[Clang][NFC] Clarify some SourceManager related code (#153527)

Static analysis flagged the columns - 1 code, it was correct but the
assumption was not obvious. I document the assumption w/ assertions.

While digging through related code I found getColumnNumber that looks
wrong at first inspection and adding parentheses makes it clearer.

1 files changed, 2 insertions, 2 deletions

diff --git a/clang/lib/Basic/SourceManager.cpp b/clang/lib/Basic/SourceManager.cpp
index 343c26e..d8ec837 100644
--- a/clang/lib/Basic/SourceManager.cpp
+++ b/clang/lib/Basic/SourceManager.cpp
@@ -1171,14 +1171,14 @@ unsigned SourceManager::getColumnNumber(FileID FID, unsigned FilePos,
         if (Buf[FilePos - 1] == '\r' || Buf[FilePos - 1] == '\n')
           --FilePos;
       }
-      return FilePos - LineStart + 1;
+      return (FilePos - LineStart) + 1;
     }
   }
 
   unsigned LineStart = FilePos;
   while (LineStart && Buf[LineStart-1] != '\n' && Buf[LineStart-1] != '\r')
     --LineStart;
-  return FilePos-LineStart+1;
+  return (FilePos - LineStart) + 1;
 }
 
 // isInvalid - Return the result of calling loc.isInvalid(), and