[clang-format] Support inheriting from more than 1 parents in the fallback case

Currently, we are unable to inherit from a chain of parent configs where the outermost parent config has `BasedOnStyle: InheritParentConfig` set. This patch adds a test case for this scenario, and adds support for it.

To illustrate, suppose we have the following directory structure:
```
- e/
  |- .clang-format (BasedOnStyle: InheritParentConfig) <-- outermost config
  |- sub/
    |- .clang-format (BasedOnStyle: InheritParentConfig)
    |- sub/
      |- .clang-format (BasedOnStyle: InheritParentConfig)
      |- code.cpp
```
Now consider what happens when we run `clang-format --style=file /e/sub/sub/code.cpp`.

Without this patch, on a release build, only the innermost config will be applied. On a debug build, clang-format crashes due to an assertion failure.
With this patch, clang-format behaves as we'd expect, applying all 3 configs.

Reviewed By: HazardyKnusperkeks, curdeius

Differential Revision: https://reviews.llvm.org/D116371

clang/lib/Format/Format.cpp

@@ -3288,6 +3288,16 @@ llvm::Expected<FormatStyle> getStyle(StringRef StyleName, StringRef FileName,
 
   auto dropDiagnosticHandler = [](const llvm::SMDiagnostic &, void *) {};
 
+  auto applyChildFormatTexts = [&](FormatStyle *Style) {
+    for (const auto &MemBuf : llvm::reverse(ChildFormatTextToApply)) {
+      auto EC = parseConfiguration(*MemBuf, Style, AllowUnknownOptions,
+                                   dropDiagnosticHandler);
+      // It was already correctly parsed.
+      assert(!EC);
+      static_cast<void>(EC);
+    }
+  };
+
   for (StringRef Directory = Path; !Directory.empty();
        Directory = llvm::sys::path::parent_path(Directory)) {
 
@@ -3330,14 +3340,7 @@ llvm::Expected<FormatStyle> getStyle(StringRef StyleName, StringRef FileName,
             return Style;
 
           LLVM_DEBUG(llvm::dbgs() << "Applying child configurations\n");
-
-          for (const auto &MemBuf : llvm::reverse(ChildFormatTextToApply)) {
-            auto Ec = parseConfiguration(*MemBuf, &Style, AllowUnknownOptions,
-                                         dropDiagnosticHandler);
-            // It was already correctly parsed.
-            assert(!Ec);
-            static_cast<void>(Ec);
-          }
+          applyChildFormatTexts(&Style);
 
           return Style;
         }
@@ -3363,17 +3366,9 @@ llvm::Expected<FormatStyle> getStyle(StringRef StyleName, StringRef FileName,
                              UnsuitableConfigFiles);
 
   if (!ChildFormatTextToApply.empty()) {
-    assert(ChildFormatTextToApply.size() == 1);
-
     LLVM_DEBUG(llvm::dbgs()
-               << "Applying child configuration on fallback style\n");
-
-    auto Ec =
-        parseConfiguration(*ChildFormatTextToApply.front(), &FallbackStyle,
-                           AllowUnknownOptions, dropDiagnosticHandler);
-    // It was already correctly parsed.
-    assert(!Ec);
-    static_cast<void>(Ec);
+               << "Applying child configurations on fallback style\n");
+    applyChildFormatTexts(&FallbackStyle);
   }
 
   return FallbackStyle;

clang/unittests/Format/FormatTest.cpp

@@ -21464,8 +21464,8 @@ TEST(FormatStyle, GetStyleOfFile) {
   ASSERT_TRUE((bool)StyleTd);
   ASSERT_EQ(*StyleTd, getLLVMStyle(FormatStyle::LK_TableGen));
 
-  // Test 9.1: overwriting a file style, when parent no file exists with no
-  // fallback style
+  // Test 9.1.1: overwriting a file style, when no parent file exists with no
+  // fallback style.
   ASSERT_TRUE(FS.addFile(
       "/e/sub/.clang-format", 0,
       llvm::MemoryBuffer::getMemBuffer("BasedOnStyle: InheritParentConfig\n"
@@ -21480,6 +21480,25 @@ TEST(FormatStyle, GetStyleOfFile) {
     return Style;
   }());
 
+  // Test 9.1.2: propagate more than one level with no parent file.
+  ASSERT_TRUE(FS.addFile("/e/sub/sub/code.cpp", 0,
+                         llvm::MemoryBuffer::getMemBuffer("int i;")));
+  ASSERT_TRUE(FS.addFile("/e/sub/sub/.clang-format", 0,
+                         llvm::MemoryBuffer::getMemBuffer(
+                             "BasedOnStyle: InheritParentConfig\n"
+                             "WhitespaceSensitiveMacros: ['FOO', 'BAR']")));
+  std::vector<std::string> NonDefaultWhiteSpaceMacros{"FOO", "BAR"};
+
+  ASSERT_NE(Style9->WhitespaceSensitiveMacros, NonDefaultWhiteSpaceMacros);
+  Style9 = getStyle("file", "/e/sub/sub/code.cpp", "none", "", &FS);
+  ASSERT_TRUE(static_cast<bool>(Style9));
+  ASSERT_EQ(*Style9, [&NonDefaultWhiteSpaceMacros] {
+    auto Style = getNoStyle();
+    Style.ColumnLimit = 20;
+    Style.WhitespaceSensitiveMacros = NonDefaultWhiteSpaceMacros;
+    return Style;
+  }());
+
   // Test 9.2: with LLVM fallback style
   Style9 = getStyle("file", "/e/sub/code.cpp", "LLVM", "", &FS);
   ASSERT_TRUE(static_cast<bool>(Style9));
@@ -21503,15 +21522,7 @@ TEST(FormatStyle, GetStyleOfFile) {
     return Style;
   }());
 
-  // Test 9.4: propagate more than one level
-  ASSERT_TRUE(FS.addFile("/e/sub/sub/code.cpp", 0,
-                         llvm::MemoryBuffer::getMemBuffer("int i;")));
-  ASSERT_TRUE(FS.addFile("/e/sub/sub/.clang-format", 0,
-                         llvm::MemoryBuffer::getMemBuffer(
-                             "BasedOnStyle: InheritParentConfig\n"
-                             "WhitespaceSensitiveMacros: ['FOO', 'BAR']")));
-  std::vector<std::string> NonDefaultWhiteSpaceMacros{"FOO", "BAR"};
-
+  // Test 9.4: propagate more than one level with a parent file.
   const auto SubSubStyle = [&NonDefaultWhiteSpaceMacros] {
     auto Style = getGoogleStyle();
     Style.ColumnLimit = 20;