add support for custom client-specific diagnostics. As a testcase, make the

rewriter emit this error if it fails to rewrite an @encode:

t.m:17:9: error: rewriter could not replace sub-expression due to macros
    c = ENC(char *)[2] + 4;
        ^~~~~~~~~~~

... where ENC is: #define ENC @encode

llvm-svn: 44498