clang-format: Support `if CONSTEXPR` if CONSTEXPR is a macro.

This is like r305666 (which added support for `if constexpr`) except
that it allows a macro name after the if.

This is slightly tricky for two reasons:

1. r305666 didn't add test coverage for all cases where it added a
   kw_constexpr, so I had to figure out what all the added cases were
   for. I now added tests for all `if constexpr` bits that didn't have
   tests. (This took a while, see e.g. https://reviews.llvm.org/D65223)

2. Parsing `if <ident> (` as an if means that `#if defined(` and
   `#if __has_include(` parse as ifs too. Add some special-case code
   to prevent this from happening where it's incorrect.

Fixes PR39248.

Differential Revision: https://reviews.llvm.org/D65227

llvm-svn: 367167