[clang] [clang] Use std::optional::value_or (NFC) (PR #109894)

[Kazu Hirata via cfe-commits]

kazutakahirata wrote:

> Next time please provide a better summary for your PR. This is super important for downstream folks debugging build breaks. In general this is important for reviews to compare what they expect with the actual diff.
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> In this case something like "replace the use of conditional operator on std::optional w/ value_or" would have been sufficient. 
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> Thank you

Ack.

https://github.com/llvm/llvm-project/pull/109894