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

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

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@44498 91177308-0d34-0410-b5e6-96231b3b80d8