[clang] [Clang] Don't give up on an unsuccessful function instantiation (PR #126723)

[Younan Zhang via cfe-commits]

zyn0217 wrote:

I looked into that weeks ago, and the reason I remembered was kind of subtle. I'll explain why the fix is necessary later - I'll need a debugger to help me recall that.

https://github.com/llvm/llvm-project/pull/126723