[clangd] Disable delayed template parsing in the main file
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.
- 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)
Subscribers: ilya-biryukov, MaskRay, jkorous, arphaman, usaxena95, cfe-commits
Differential Revision: https://reviews.llvm.org/D78848