[clangd] Run semaCodeComplete only with a preamble

Summary:
It is used by code completion and signature help. Code completion
already uses a special no-compile mode for missing preambles, so this change is
a no-op for that.

As for signature help, it already blocks for a preamble and missing it implies
clang has failed to parse the preamble and retrying it in signature help likely
will fail again. And even if it doesn't, request latency will be too high to be
useful as parsing preambles is expensive.

Reviewers: sammccall

Subscribers: ilya-biryukov, MaskRay, jkorous, arphaman, usaxena95, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D77204