[clangd] parse all make_unique-like functions in preamble

I am working on support for forwarding parameter names in make_unique-like functions, first for inlay hints, later maybe for signature help.
For that to work generically, I'd like to parse all of these functions in the preamble. Not sure how this impacts performance on large codebases though.

Reviewed By: sammccall

Differential Revision: https://reviews.llvm.org/D124688