Prepare dual-mode compiler plugin feature

...which can act as either a rewriter or a non-rewriter that emits warnings.

Also added COMPILER_PLUGIN_WARNINGS_ONLY=X to demote warnings from plugin X from
errors to warnings, even under --enable-werror.

Change-Id: I05361936240a890515c6bba2459565417c1746b7

compilerplugins/clang/postfixincrementfix.cxx

@@ -20,8 +20,8 @@ Change all postfix ++ operators of non-trivial types to prefix if possible.
 namespace loplugin
 {
 
-PostfixIncrementFix::PostfixIncrementFix( CompilerInstance& compiler, Rewriter& rewriter )
-    : RewritePlugin( compiler, rewriter )
+PostfixIncrementFix::PostfixIncrementFix( const InstantiationData& data )
+    : RewritePlugin( data )
     {
     }