[Basic] Add missing `const` qualifier (NFC)

[thegameg]

Summary

The member function is a predicate, and doesn't apply any changes on the object.

[vsk]

Makes sense. isMacroID and isInSystemHeader are both already marked const.

[thegameg]

Thanks @vsk. As I don't have commit access, is it possible to commit this for me, please? Thanks.

cfe/trunk/include/clang/Basic/SourceManager.h

public:
   }
 
   /// \brief Returns if a SourceLocation is in an "extern C" system header.
   bool isInExternCSystemHeader(SourceLocation Loc) const {
     return getFileCharacteristic(Loc) == SrcMgr::C_ExternCSystem;
   }
 
   /// \brief Returns whether \p Loc is expanded from a macro in a system header.
-  bool isInSystemMacro(SourceLocation loc) {
+  bool isInSystemMacro(SourceLocation loc) const {
     return loc.isMacroID() && isInSystemHeader(getSpellingLoc(loc));
   }
 
   /// \brief The size of the SLocEntry that \p FID represents.
   unsigned getFileIDSize(FileID FID) const;
 
   /// \brief Given a specific FileID, returns true if \p Loc is inside that
   /// FileID chunk and sets relative offset (offset of \p Loc from beginning