[clangd] Speculative code completion index request before Sema is run.

Summary:
For index-based code completion, send an asynchronous speculative index
request, based on the index request for the last code completion on the same
file and the filter text typed before the cursor, before sema code completion
is invoked. This can reduce the code completion latency (by roughly latency of
sema code completion) if the speculative request is the same as the one
generated for the ongoing code completion from sema. As a sequence of code
completions often have the same scopes and proximity paths etc, this should be
effective for a number of code completions.

Trace with speculative index request:{F6997544}

Reviewers: hokein, ilya-biryukov

Reviewed By: ilya-biryukov

Subscribers: javed.absar, jfb, MaskRay, jkorous, arphaman, kadircet, cfe-commits

Differential Revision: https://reviews.llvm.org/D50962

llvm-svn: 340604