[clangd] Ignore sema code complete callback with recovery context.

Summary:
Sema code complete in the recovery mode is generally useless. For many
cases, sema first completes in recovery context and then recovers to more useful
context, in which it's favorable to ignore results from recovery (as results are
often bad e.g. all builtin symbols and top-level symbols). There is also case
where only sema would fail to recover e.g. completions in excluded #if block.
Sema would try to give results, but the results are often useless (see the updated
excluded #if block test).

Reviewers: sammccall, ilya-biryukov

Subscribers: MaskRay, jkorous, cfe-commits

Differential Revision: https://reviews.llvm.org/D49175

llvm-svn: 336801