[meta] Reduce build time of llvm+clang

[RKSimon]

Bugzilla Link	34591
Version	unspecified
OS	Windows NT
Depends On	#28596 #35303 #33607
CC	@filcab,@jryans,@rotateright,@yuanfang-chen

Extended Description

Meta bug to keep track of possible ways to reduce the build/rebuild time for llvm+clang

[rotateright]

No idea if this has any impact, but we don't produce optimal code for dyn_cast - bug 28430. Given that's a fundamental code construct throughout clang/llvm, I thought it might be relevant to compile-time.

I was chasing that, but the last step I tried had problems:
llvm/llvm-bugzilla-archive#32082
https://reviews.llvm.org/D29999

[nico]

mentioned in issue #35303