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/bodynotinblock.cxx

@@ -29,8 +29,8 @@ For example:
 Here either both statements should be inside {} or the second statement in indented wrong.
 */
 
-BodyNotInBlock::BodyNotInBlock( CompilerInstance& compiler )
-    : Plugin( compiler )
+BodyNotInBlock::BodyNotInBlock( const InstantiationData& data )
+    : Plugin( data )
     {
     }