[Clang] Fix diagnostics formatting

[xtkoba]

Summary

When a #warning directive is followed by tokens containing non-ASCII characters encoded in UTF-8, a broken string can be printed out as a diagnostic message on some systems. This is because the current code assumes char to be signed char, whereas on some systems char is in fact unsigned char.

[xtkoba]

The issue was first reported by @dev-bz at GitHub in https://github.com/termux/termux-packages/issues/9619.

[aaron.ballman]

Thank you for this! Is there a way to add test coverage for this change? Also, this should come with a release note as it's fixing a bug someone reported.

clang/lib/Basic/Diagnostic.cpp

FormatDiagnostic(const char *DiagStr, const char *DiagEnd,
                  SmallVectorImpl<char> &OutStr) const {
   // When the diagnostic string is only "%0", the entire string is being given
   // by an outside source.  Remove unprintable characters from this string
   // and skip all the other string processing.
   if (DiagEnd - DiagStr == 2 &&
       StringRef(DiagStr, DiagEnd - DiagStr).equals("%0") &&
       getArgKind(0) == DiagnosticsEngine::ak_std_string) {
     const std::string &S = getArgStdStr(0);
-    for (char c : S) {
+    for (signed char c : S) {
       if (llvm::sys::locale::isPrint(c) || c == '\t') {
         OutStr.push_back(c);
       }
     }
     return;
   }
 
   /// FormattedArgs - Keep track of all of the arguments formatted by