[clang][Diagnostics][NFC] Don't create oversized CaretLine

[tbaeder]

Summary

Instead of creating a CaretLine the size of the SourceLine, just leave
it empty at first, let HighlightRange resize it to fit all the ~, then
resize it to fit the ^. Then we can save ourselves the work to remove
the trailing whitespace again.

[aaron.ballman]

LGTM!

clang/lib/Frontend/TextDiagnostic.cpp

while (!Line.empty() && Line.back() == '\0' &&
       Line = Line.drop_back();
 
     // Copy the line of code into an std::string for ease of manipulation.
     std::string SourceLine(Line.begin(), Line.end());
 
     // Build the byte to column map.
     const SourceColumnMap sourceColMap(SourceLine, DiagOpts->TabStop);
 
-    // Create a line for the caret that is filled with spaces that is the same
-    // number of columns as the line of source code.
-    std::string CaretLine(sourceColMap.columns(), ' ');
-
+    std::string CaretLine;
     // Highlight all of the characters covered by Ranges with ~ characters.
     for (const auto &I : Ranges)
       highlightRange(I, LineNo, FID, sourceColMap, CaretLine, SM, LangOpts);
 
     // Next, insert the caret itself.
     if (CaretLineNo == LineNo) {
-      CaretColNo = sourceColMap.byteToContainingColumn(CaretColNo - 1);
-      if (CaretLine.size() < CaretColNo + 1)
-        CaretLine.resize(CaretColNo + 1, ' ');
-      CaretLine[CaretColNo] = '^';
+      size_t Col = sourceColMap.byteToContainingColumn(CaretColNo - 1);
+      CaretLine.resize(std::max(Col + 1, CaretLine.size()), ' ');
+      CaretLine[Col] = '^';
     }
 
     std::string FixItInsertionLine = buildFixItInsertionLine(
         FID, LineNo, sourceColMap, Hints, SM, DiagOpts.get());
 
     // If the source line is too long for our terminal, select only the
     // "interesting" source region within that line.
     unsigned Columns = DiagOpts->MessageLength;
     if (Columns)
       selectInterestingSourceRegion(SourceLine, CaretLine, FixItInsertionLine,
                                     Columns, sourceColMap);
 
     // If we are in -fdiagnostics-print-source-range-info mode, we are trying
     // to produce easily machine parsable output.  Add a space before the
     // source line and the caret to make it trivial to tell the main diagnostic
     // line from what the user is intended to see.
     if (DiagOpts->ShowSourceRanges) {
       SourceLine = ' ' + SourceLine;
       CaretLine = ' ' + CaretLine;
     }
 
-    // Finally, remove any blank spaces from the end of CaretLine.
-    while (!CaretLine.empty() && CaretLine[CaretLine.size() - 1] == ' ')
-      CaretLine.erase(CaretLine.end() - 1);
-
     // Emit what we have computed.
     emitSnippet(SourceLine, MaxLineNoDisplayWidth, DisplayLineNo);
 
     if (!CaretLine.empty()) {
       indentForLineNumbers();
       if (DiagOpts->ShowColors)
         OS.changeColor(caretColor, true);
       OS << CaretLine << '\n';