[clang-format] Assert that filenames are not empty

Summary:
Adds asserts to catch empty filenames, which otherwise will cause a crash in SourceManager.
The clang-format tool now outputs an error if an empty filename is used.
Fixes bug: 34667

Reviewers: krasimir, djasper, MyDeveloperDay

Reviewed By: MyDeveloperDay

Subscribers: cfe-commits

Patch by: @jr

Tags: #clang-format, #clang

Differential Revision: https://reviews.llvm.org/D56345

Author: mydeveloperday

clang/tools/clang-format/ClangFormat.cpp

@@ -390,6 +390,10 @@ static bool format(StringRef FileName) {
   if (fillRanges(Code.get(), Ranges))
     return true;
   StringRef AssumedFileName = (FileName == "-") ? AssumeFileName : FileName;
+  if (AssumedFileName.empty()) {
+    llvm::errs() << "error: empty filenames are not allowed\n";
+    return true;
+  }
 
   llvm::Expected<FormatStyle> FormatStyle =
       getStyle(Style, AssumedFileName, FallbackStyle, Code->getBuffer());