The root source of other odd bugs.
We performed a hack in CodeGen/Coroutines. But we didn't recognize that
the CodeGen is a consumer of AST. The CodeGen shouldn't change AST in
any ways. It'll break the assumption about the ASTConsumer in Clang's
framework, which may break any other clang-based tools which depends on
multiple consumers to work together.
The fix here is simple. But I am not super happy about the test. It is
too specific and verbose. We can remove this if we can get the signature
of the AST in ASTContext.
