const improvement

Change-Id: I03815f33117a2927b1882d221f159a5de8d3ba78

compilerplugins/clang/plugin.cxx

@@ -29,7 +29,7 @@ Plugin::Plugin( const InstantiationData& data )
     {
     }
 
-DiagnosticBuilder Plugin::report( DiagnosticsEngine::Level level, StringRef message, SourceLocation loc )
+DiagnosticBuilder Plugin::report( DiagnosticsEngine::Level level, StringRef message, SourceLocation loc ) const
     {
     return handler.report( level, name, message, compiler, loc );
     }

compilerplugins/clang/plugin.hxx

@@ -62,7 +62,7 @@ class Plugin
         // Returns location right after the end of the token that starts at the given location.
         SourceLocation locationAfterToken( SourceLocation location );
     protected:
-        DiagnosticBuilder report( DiagnosticsEngine::Level level, StringRef message, SourceLocation loc = SourceLocation());
+        DiagnosticBuilder report( DiagnosticsEngine::Level level, StringRef message, SourceLocation loc = SourceLocation()) const;
         bool ignoreLocation( SourceLocation loc );
         bool ignoreLocation( const Decl* decl );
         bool ignoreLocation( const Stmt* stmt );