[Lex] Use SourceLocation::UIntTy for the MacroDefStart, NFC

Avoid using the underlying type, and be more consistent -- MacroDefStart stores
the result of SourceManager::getNextLocalOffset() which returns the
SourceLocation::UIntTy.

Author: hokein

clang/include/clang/Lex/TokenLexer.h

@@ -65,7 +65,7 @@ class TokenLexer {
 
   /// The offset of the macro expansion in the
   /// "source location address space".
-  unsigned MacroStartSLocOffset;
+  SourceLocation::UIntTy MacroStartSLocOffset;
 
   /// Location of the macro definition.
   SourceLocation MacroDefStart;