[clang] [clang][DebugInfo] Store current location metadata in CGDebugInfo (PR #193918)

[Cullen Rhodes via cfe-commits]

c-rhodes wrote:

> LGTM. This means we'll be leaning on state a little more, with the risk of something getting stale, but I guess that's the trade-off for these optimisations.
> 
> An entirely optional extra bit of validation would be building stage2-clang with/without this patch and seeing whether it builds an identical binary -- that usually sets my mind at rest that there aren't any other sneek paths that are getting stale state. (Not necessary to land IMO).

stage2-clang build is a good idea. With a few config tweaks for reproducibility (`-DLLVM_APPEND_VC_REV=false`, `-ffile-prefix-map`/`-fdebug-prefix-map`) I'm getting identical binaries. I also added instrumentation to verify the cache is kicking in and there's lot of hits.

https://github.com/llvm/llvm-project/pull/193918