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

[Matheus Izvekov via cfe-commits]

mizvekov wrote:

>From my testing in compiler explorer, GCC and Clang always produce programs that return 1 as far back as we can test.
ICC and MSVC do otherwise, they return 0.

So I think this supports the notion that this should be a dialect, and we have to wire defaults differently in MSVC mode as well.

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