[pseudo] Eliminate the dangling-else syntax ambiguity.

[pseudo] Eliminate the dangling-else syntax ambiguity.

- the grammar ambiguity is eliminated by a guard;
- modify the guard function signatures, now all parameters are folded in
  to a single object, avoid a long parameter list (as we will add more
  parameters in the near future);

Reviewed By: sammccall

Differential Revision: https://reviews.llvm.org/D130160