[clangd] Store preamble macros in dynamic index.

Summary:
Pros:
o Loading macros from preamble for every completion is slow (see profile).
o Calculating macro USR is also slow (see profile).
o Sema can provide a lot of macro completion results (e.g. when filter is empty,
60k for some large TUs!).

Cons:
o Slight memory increase in dynamic index (~1%).
o Some extra work during preamble build (should be fine as preamble build and
indexAST is way slower).

Before:
{F7195645}

After:
{F7195646}

Reviewers: ilya-biryukov, sammccall

Reviewed By: sammccall

Subscribers: MaskRay, jkorous, arphaman, kadircet, cfe-commits

Differential Revision: https://reviews.llvm.org/D52078

llvm-svn: 342529