[llvm] [IR] Cache llvm.module.flags metadata in Module (PR #103410)

[Alexis Engelke via llvm-commits]

aengelke wrote:

[c-t-t](http://llvm-compile-time-tracker.com/compare.php?from=ae466a61d39454d9f311e2e4b624e256bbd5d18b&to=e9d0ffff517232c02c7ee74da2ecb32a119d7dee&stat=instructions%3Au), improvement is just barely measurable. In my local profiles, getModuleFlag shows up at 0.21%, with 0.15% being spent in StringMap + hashing.

https://github.com/llvm/llvm-project/pull/103410