[clang] [clang][Modules] Reporting Errors for Duplicating Link Declarations in `modulemap`s (PR #148959)

[Qiongsi Wu via cfe-commits]

qiongsiwu wrote:

> I think a hard error is a bit too much initially, maybe let's introduce a warning first and then tighten the screws later?

Thanks for the comment! While I agree that having a hard error initially may be harsh, I think we should go with an error for two reasons:

1. I am not seeing duplicates in SDKs.
2. Having an error will prevent duplicates getting accidentally added to the SDKs. 

I'd like to keep this an error if there are no objections. 

https://github.com/llvm/llvm-project/pull/148959