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/unusedvariablecheck.cxx

@@ -38,8 +38,8 @@ SAL_WARN_UNUSED (see e.g. OUString). For external classes such as std::vector
 that cannot be edited there is a manual list below.
 */
 
-UnusedVariableCheck::UnusedVariableCheck( CompilerInstance& compiler )
-    : Plugin( compiler )
+UnusedVariableCheck::UnusedVariableCheck( const InstantiationData& data )
+    : Plugin( data )
     {
     }