[clang][Diagnostics] Simplify emitSnippet()

[tbaeder]

Summary

Don't try to minimize the times we invoke operator<< on the output
stream by keeping a ToPrint string around. Instead, just print the
characters as we iterate over them.

[aaron.ballman]

I think this logic is the same (it's a bit tricky to tell, so it's unfortunate that precommit CI isn't running for this to help validate), so LGTM.

clang/lib/Frontend/TextDiagnostic.cpp

if (MaxLineNoDisplayWidth > 0) {
     unsigned LineNoDisplayWidth = getNumDisplayWidth(LineNo);
     OS << ' ';
     for (unsigned I = LineNoDisplayWidth; I < MaxLineNoDisplayWidth; ++I)
       OS << ' ';
     OS << LineNo;
     OS << " | ";
   }
 
+  // Print the source line one character at a time.
   bool PrintReversed = false;
-  std::string ToPrint;
   size_t I = 0;
   while (I < SourceLine.size()) {
     auto [Str, WasPrintable] =
         printableTextForNextCharacter(SourceLine, &I, DiagOpts->TabStop);
 
-    if (DiagOpts->ShowColors && WasPrintable == PrintReversed) {
+    // Toggle inverted colors on or off for this character.
+    if (DiagOpts->ShowColors) {
+      if (WasPrintable == PrintReversed) {
+        PrintReversed = !PrintReversed;
         if (PrintReversed)
           OS.reverseColor();
-      OS << ToPrint;
-      ToPrint.clear();
-      if (DiagOpts->ShowColors)
+        else
           OS.resetColor();
       }
-
-    PrintReversed = !WasPrintable;
-    ToPrint += Str;
+    }
+    OS << Str;
   }
 
-  if (PrintReversed && DiagOpts->ShowColors)
-    OS.reverseColor();
-  OS << ToPrint;
-  if (PrintReversed && DiagOpts->ShowColors)
+  if (DiagOpts->ShowColors)
     OS.resetColor();
 
   OS << '\n';
 }
 
 void TextDiagnostic::emitParseableFixits(ArrayRef<FixItHint> Hints,
                                          const SourceManager &SM) {
   if (!DiagOpts->ShowParseableFixits)