[clang] [C] Disable use of NRVO (PR #101038)

[Aaron Ballman via cfe-commits]

AaronBallman wrote:

> > Should we try to convince at least the GCC folks that they're getting this wrong too?
> 
> It does feel like a wider conversation is necessary, yeah. The fact that everybody is doing this even on x86_64 where it's pretty incontrovertibly forbidden seems significant.

CC @pinskia for awareness

https://github.com/llvm/llvm-project/pull/101038