[clangd] Force delayed-template-parsing off in code completion.

[clangd] Force delayed-template-parsing off in code completion.

Summary:
It prevents code completion entirely in affected method bodies.
The main reason it's turned on is for compatibility with headers, so we turn it
off for the main file only. This is allowed because it's a compatible langopt.

Fixes https://github.com/clangd/clangd/issues/302

Reviewers: kadircet

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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D77176