[clang] [Clang] Instantiate constexpr function when they are needed. (PR #173537)

[James Y Knight via cfe-commits]

jyknight wrote:

> The plan is to do a follow up once this lands ( as mentioned in the description)

Ah, sorry! I saw that, but I didn't understand that my example was an instance of the not-expected-to-be-fixed issue, rather than an example of the expected-to-be-fixed issue.

At least for me, it was unclear what the distinguishing factors are between "fixed in this PR" vs "deferred to followup PR" were.

https://github.com/llvm/llvm-project/pull/173537