[clang] [Clang] Instantiate functions from constant evaluation. (PR #205557)

[Aaron Ballman via cfe-commits]

AaronBallman wrote:

> Can we put this behind a flag for now? I don't want to block progress here, but it also seems likely to cause problems, and I don't want it bouncing in and out of the tree.

I think a flag makes a fair amount of sense, particularly because I think we're still going to find corner cases that need adjusting. I think it'd make sense for the flag to start out as opt-in, then once we think we're ready, transition to opt-out, and hopefully we can eventually deprecate and remove the flag.

https://github.com/llvm/llvm-project/pull/205557