[clang][CodeComplete] Dont perform fallback completion for incomplete member ref

Summary:
Clang performs expression based completion whenever it can't figure out
base of a member reference expression. It might be quite confusing in cases like
incomplete types. This patch disables that fallback.

Unfortunately `ParsePostfixExpressionSuffix` is quite tangled and this patch
adds more to it.

Reviewers: sammccall

Subscribers: cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D77570