[clang][parser] Unify rejecting (non) decl stmts with gnu attributes

The comment here was introduced in
a3e01cf8 and suggests that we should
handle declaration statements and non-declaration statements the same,
but don't because ProhibitAttributes() can't handle GNU attributes. That
has recently changed, so remove the comment and handle all statements
the same.

Differential Revision: https://reviews.llvm.org/D99936