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

[Aaron Ballman via cfe-commits]

https://github.com/AaronBallman commented:

Because this is intending to help avoid a significant library layering issue, I've included @zygoloid @chandlerc and @rjmccall as reviewers because they've got non-C or C++ downstream uses which are the most likely to be surprised by the AST layer now having the potential to involve semantic analysis that can mutate the AST. Others are of course welcome to comment on the design (I've left a few of my own thoughts in the PR).

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