[clang-format] [PR42014,PR52021] don't let clang-format assert/crash when file being formatted is read-only/locked

[MyDeveloperDay]

Summary

This is a bug which gets reported from time to time and we've had multiple attempts to fix it, but don't want to fix it by adding frontEnd to the mix.

D68554: [clang-format] Proposal for clang-format to give compiler style warnings
D90121: clang-format: Add a consumer to diagnostics engine
D69854: [clang-format] [RELAND] Remove the dependency on frontend

This patch aim to find a trivial, but not that sophisticated way of emitting the error without the additional impact of adding libFrontEnd to clang-format.

See D90121: clang-format: Add a consumer to diagnostics engine for analysis of why we don't want those previous attempts

We can now see this:

$ clang-format -i test1.cpp
clang-format error:unable to rename temporary 'test1.cpp-4e5b1ac5' to output file 'test1.cpp': 'permission denied'

Rather than

Assertion failed: getClient() && "DiagnosticClient not set!", file C:\llvm-project\clang\lib\Basic\Diagnostic.cpp, line 522
PLEASE submit a bug report to https://bugs.llvm.org/ and include the crash backtrace.
Stack dump:
0.      Program arguments: C:\\build_ninja\\bin\\clang-format.exe -i test1.cpp
 #0 0x00007ff63cd7244c (C:\build_ninja\bin\clang-format.exe+0x12244c)
 #1 0x00007fff3db28e05 (C:\WINDOWS\SYSTEM32\ucrtbased.dll+0xa8e05)
...
 #6 0x00007fff3db3158f (C:\WINDOWS\SYSTEM32\ucrtbased.dll+0xb158f)
 #7 0x00007ff63ce6d4ff (C:\build_ninja\bin\clang-format.exe+0x21d4ff)
 #8 0x00007ff63ce7dcf6 (C:\build_ninja\bin\clang-format.exe+0x22dcf6)
....
#17 0x00007ff63d16457e (C:\build_ninja\bin\clang-format.exe+0x51457e)
#18 0x00007fff9b7f7034 (C:\WINDOWS\System32\KERNEL32.DLL+0x17034)
#19 0x00007fff9ce02651 (C:\WINDOWS\SYSTEM32\ntdll.dll+0x52651)

Not sure how I can easily unit test this, as I had to use Excel to lock the file!

clang/tools/clang-format/ClangFormat.cpp

static void outputXML(const Replacements &Replaces,
   if (Cursor.getNumOccurrences() != 0)
     outs() << "<cursor>" << FormatChanges.getShiftedCodePosition(CursorPosition)
            << "</cursor>\n";
 
   outputReplacementsXML(Replaces);
   outs() << "</replacements>\n";
 }
 
+class ClangFormatDiagConsumer : public DiagnosticConsumer {
+  virtual void anchor() {}
+
+  void HandleDiagnostic(DiagnosticsEngine::Level DiagLevel,
+                        const Diagnostic &Info) override {
+
+    SmallVector<char, 16> vec;
+    Info.FormatDiagnostic(vec);
+    errs() << "clang-format error:" << vec << "\n";
+  }
+};
+
 // Returns true on error.
 static bool format(StringRef FileName) {
   if (!OutputXML && Inplace && FileName == "-") {
     errs() << "error: cannot use -i when reading from stdin.\n";
     return false;
   }
   // On Windows, overwriting a file with an open file mapping doesn't work,
   // so read the whole file into memory when formatting in-place.
if (DryRun) {
       return emitReplacementWarnings(Replaces, AssumedFileName, Code);
     } else {
       outputXML(Replaces, FormatChanges, Status, Cursor, CursorPosition);
     }
   } else {
     IntrusiveRefCntPtr<llvm::vfs::InMemoryFileSystem> InMemoryFileSystem(
         new llvm::vfs::InMemoryFileSystem);
     FileManager Files(FileSystemOptions(), InMemoryFileSystem);
+
+    IntrusiveRefCntPtr<DiagnosticOptions> DiagOpts(new DiagnosticOptions());
+    ClangFormatDiagConsumer IgnoreDiagnostics;
     DiagnosticsEngine Diagnostics(
-        IntrusiveRefCntPtr<DiagnosticIDs>(new DiagnosticIDs),
-        new DiagnosticOptions);
+        IntrusiveRefCntPtr<DiagnosticIDs>(new DiagnosticIDs), &*DiagOpts,
+        &IgnoreDiagnostics, false);
     SourceManager Sources(Diagnostics, Files);
     FileID ID = createInMemoryFile(AssumedFileName, *Code, Sources, Files,
                                    InMemoryFileSystem.get());
     Rewriter Rewrite(Sources, LangOptions());
     tooling::applyAllReplacements(Replaces, Rewrite);
     if (Inplace) {
       if (Rewrite.overwriteChangedFiles())
         return true;