[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.

clang/lib/Basic/SourceManager.cpp

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 @@
         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

clang/lib/Frontend/TextDiagnostic.cpp

diff --git a/clang/lib/Frontend/TextDiagnostic.cpp b/clang/lib/Frontend/TextDiagnostic.cpp
index ccdd59d..5888571 100644
--- a/clang/lib/Frontend/TextDiagnostic.cpp
+++ b/clang/lib/Frontend/TextDiagnostic.cpp
@@ -1095,6 +1095,8 @@
 
     unsigned StartColumn = SM.getExpansionColumnNumber(Begin);
     unsigned EndColumn = SM.getExpansionColumnNumber(End);
+    assert(StartColumn && "StartColumn must be valid, 0 is invalid");
+    assert(EndColumn && "EndColumn must be valid, 0 is invalid");
     if (R.isTokenRange())
       EndColumn += Lexer::MeasureTokenLength(End, SM, LangOpts);