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

[John McCall via cfe-commits]

rjmccall 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.

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