...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
It's possible to get the latter from the former, and the former
is useful for other things too (access to the preprocessor, for example).
Change-Id: I708d709129fd3a35bf7c63da4de09c2e696b382d