[clang] [Modules] No transitive source location change (PR #86912)

[Chuanqi Xu via cfe-commits]

ChuanqiXu9 wrote:

> The changes LGTM, don't want to block this on my remaining nits.

Thanks for reviewing this.

> 
> I believe @Bigcheese wanted to test test impact on PCM size on our side before this lands.

I've rebased this with main. I'll wait for the results from @Bigcheese 

https://github.com/llvm/llvm-project/pull/86912