[clangd] Disable delayed template parsing in the main file

Summary:
This is on by default in windows and breaks most features in template bodies.
We'd already disabled it in code completion, now disable it for building ASTs.

Potential regressions:
 - we may give spurious errors where files with templates relying on delayed
   parsing are directly opened
 - we may misparse such template bodies that are instantiated (and therefore
   *were* previously parsed)

Still *probably* a win overall. Avoiding the regressions entirely would be
substantial work and we don't have plans for it now.

Fixes https://github.com/clangd/clangd/issues/302 (again)

Reviewers: kadircet

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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D78848