[pseudo] Implement a guard to determine function declarator.

This eliminates some simple-declaration/function-definition false
parses.

- implement a function to determine whether a declarator ForestNode is a
  function declarator;
- extend the standard declarator to two guarded function-declarator and
  non-function-declarator nonterminals;

Differential Revision: https://reviews.llvm.org/D129222