[pseudo] Reorganize CXX.h enums

- Place rules under rule::lhs::rhs__rhs__rhs
- Change mangling of keywords to ALL_CAPS (needed to turn keywords that appear
  alone on RHS into valid identifiers)
- Make enums implicitly convertible to underlying type (though still scoped,
  using alias tricks)

In principle this lets us exhaustively write a switch over all rules of a NT:
  switch ((rule::declarator)N->rule()) {
    case rule::declarator::noptr_declarator:
      ...
  }
In practice we don't do this anywhere yet as we're often switching over multiple
nonterminal kinds at once.

Differential Revision: https://reviews.llvm.org/D130414