Revert "Frontend: Sink named pipe logic from CompilerInstance down to FileManager"

This reverts commit 3b18a594c7, since
apparently this doesn't work everywhere. E.g.,
clang-x86_64-debian-fast/3889
(http://lab.llvm.org:8011/#/builders/109/builds/3889) gives me:
```
+ : 'RUN: at line 8'
+ /b/1/clang-x86_64-debian-fast/llvm.obj/bin/clang -x c /dev/fd/0 -E
+ cat /b/1/clang-x86_64-debian-fast/llvm.src/clang/test/Misc/dev-fd-fs.c
fatal error: file '/dev/fd/0' modified since it was first processed
1 error generated.
```

clang/lib/Basic/FileManager.cpp

@@ -489,7 +489,7 @@ FileManager::getBufferForFile(const FileEntry *Entry, bool isVolatile,
   uint64_t FileSize = Entry->getSize();
   // If there's a high enough chance that the file have changed since we
   // got its size, force a stat before opening it.
-  if (isVolatile || Entry->isNamedPipe())
+  if (isVolatile)
     FileSize = -1;
 
   StringRef Filename = Entry->getName();

clang/lib/Frontend/CompilerInstance.cpp

@@ -858,8 +858,30 @@ bool CompilerInstance::InitializeSourceManager(const FrontendInputFile &Input,
     }
     FileEntryRef File = *FileOrErr;
 
-    SourceMgr.setMainFileID(
-        SourceMgr.createFileID(File, SourceLocation(), Kind));
+    // The natural SourceManager infrastructure can't currently handle named
+    // pipes, but we would at least like to accept them for the main
+    // file. Detect them here, read them with the volatile flag so FileMgr will
+    // pick up the correct size, and simply override their contents as we do for
+    // STDIN.
+    if (File.getFileEntry().isNamedPipe()) {
+      auto MB =
+          FileMgr.getBufferForFile(&File.getFileEntry(), /*isVolatile=*/true);
+      if (MB) {
+        // Create a new virtual file that will have the correct size.
+        const FileEntry *FE =
+            FileMgr.getVirtualFile(InputFile, (*MB)->getBufferSize(), 0);
+        SourceMgr.overrideFileContents(FE, std::move(*MB));
+        SourceMgr.setMainFileID(
+            SourceMgr.createFileID(FE, SourceLocation(), Kind));
+      } else {
+        Diags.Report(diag::err_cannot_open_file) << InputFile
+                                                 << MB.getError().message();
+        return false;
+      }
+    } else {
+      SourceMgr.setMainFileID(
+          SourceMgr.createFileID(File, SourceLocation(), Kind));
+    }
   } else {
     llvm::ErrorOr<std::unique_ptr<llvm::MemoryBuffer>> SBOrErr =
         llvm::MemoryBuffer::getSTDIN();