[clang-format] Print the names of unfound files in error messages

Fixes #113631.

Author: owenca

clang/test/Format/error-unfound-files.cpp

@@ -0,0 +1,7 @@
+// RUN: rm -f a.c b.c
+
+// RUN: not clang-format a.c b.c 2>&1 | FileCheck %s
+// CHECK: a.c:
+// CHECK-NEXT: b.c:
+
+// rm -f a.c b.c

clang/tools/clang-format/ClangFormat.cpp

@@ -419,7 +419,7 @@ static bool format(StringRef FileName, bool ErrorOnIncompleteFormat = false) {
           ? MemoryBuffer::getFileAsStream(FileName)
           : MemoryBuffer::getFileOrSTDIN(FileName, /*IsText=*/true);
   if (std::error_code EC = CodeOrErr.getError()) {
-    errs() << EC.message() << "\n";
+    errs() << FileName << ": " << EC.message() << "\n";
     return true;
   }
   std::unique_ptr<llvm::MemoryBuffer> Code = std::move(CodeOrErr.get());