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/literalalternative.cxx

@@ -21,7 +21,7 @@ class LiteralAlternative:
     public RecursiveASTVisitor<LiteralAlternative>, public loplugin::Plugin
 {
 public:
-    explicit LiteralAlternative(CompilerInstance & compiler): Plugin(compiler) {}
+    explicit LiteralAlternative(InstantiationData const & data): Plugin(data) {}
 
     virtual void run() override { TraverseDecl(compiler.getASTContext().getTranslationUnitDecl()); }