[Basic] getColumnNumber returns location of CR+LF on Windows

[chh]

Summary

When fixing a Clang-Tidy bug in D31406,
http://bugs.llvm.org/show_bug.cgi?id=32402,
reuse of FileID enabled the missing highlightRange function.
Assertion in highlightRange failed because the end-of-range column
number was 2 + the last column of a line on Windows.
This fix is required to enable D31406.

[alexfh]

LG with one nit.

Thank you for tracking down this bug and fixing it!

lib/Basic/SourceManager.cpp

1154

I'd move this line before the outermost if and remove the identical line below.

lib/Basic/SourceManager.cpp

unsigned SourceManager::getColumnNumber(FileID FID, unsigned FilePos,
 
   // It is okay to request a position just past the end of the buffer.
   if (FilePos > MemBuf->getBufferSize()) {
     if (Invalid)
       *Invalid = true;
     return 1;
   }
 
+  const char *Buf = MemBuf->getBufferStart();
   // See if we just calculated the line number for this FilePos and can use
   // that to lookup the start of the line instead of searching for it.
   if (LastLineNoFileIDQuery == FID &&
       LastLineNoContentCache->SourceLineCache != nullptr &&
       LastLineNoResult < LastLineNoContentCache->NumLines) {
     unsigned *SourceLineCache = LastLineNoContentCache->SourceLineCache;
     unsigned LineStart = SourceLineCache[LastLineNoResult - 1];
     unsigned LineEnd = SourceLineCache[LastLineNoResult];
-    if (FilePos >= LineStart && FilePos < LineEnd)
+    if (FilePos >= LineStart && FilePos < LineEnd) {
+      // LineEnd is the LineStart of the next line.
+      // A line ends with separator LF or CR+LF on Windows.
+      // FilePos might point to the last separator,
+      // but we need a column number at most 1 + the last column.
+      if (FilePos + 1 == LineEnd && FilePos > LineStart) {
+        if (Buf[FilePos - 1] == '\r' || Buf[FilePos - 1] == '\n')
+          --FilePos;
+      }
       return FilePos - LineStart + 1;
     }
+  }
 
-  const char *Buf = MemBuf->getBufferStart();
   unsigned LineStart = FilePos;
   while (LineStart && Buf[LineStart-1] != '\n' && Buf[LineStart-1] != '\r')
     --LineStart;
   return FilePos-LineStart+1;
 }
 
 // isInvalid - Return the result of calling loc.isInvalid(), and
 // if Invalid is not null, set its value to same.