FileError: Provide a way to retrieve the underlying error string without the file name

For use with APIs that want to report the file name in a different
syntactic form, have other knowledge of the filename, etc.

llvm/include/llvm/Support/Error.h

@@ -1266,6 +1266,13 @@ public:
     Err->log(OS);
   }
 
+  std::string messageWithoutFileInfo() const {
+    std::string Msg;
+    raw_string_ostream OS(Msg);
+    Err->log(OS);
+    return OS.str();
+  }
+
   StringRef getFileName() { return FileName; }
 
   Error takeError() { return Error(std::move(Err)); }

llvm/unittests/Support/ErrorTest.cpp

@@ -958,8 +958,13 @@ TEST(Error, FileErrorTest) {
                          "'file2.bin': CustomError {42}"),
             0);
 
-  Error FE5 = createFileError("", make_error<CustomError>(1));
-  EXPECT_EQ(toString(std::move(FE5)).compare("'': CustomError {1}"), 0);
+  Error FE5 = createFileError("", make_error<CustomError>(5));
+  EXPECT_EQ(toString(std::move(FE5)).compare("'': CustomError {5}"), 0);
+
+  Error FE6 = createFileError("unused", make_error<CustomError>(6));
+  handleAllErrors(std::move(FE6), [](std::unique_ptr<FileError> F) {
+    EXPECT_EQ(F->messageWithoutFileInfo(), "CustomError {6}");
+  });
 }
 
 enum class test_error_code {