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

[Aaron Ballman via cfe-commits]

AaronBallman wrote:

I'm a bit less clear on what to do now. It sounds like nobody is opposed to a language dialect flag (default off) so users can retain the existing behavior. But it also sounds like perhaps the correct approach to this is to modify codegen and not the NRVO markings when building the AST. (Then again, wouldn't we want to drop that bit from the AST in C anyway because we wouldn't want AST consumers to presume that NRVO is valid, such as one-off AST matchers for use in clang-tidy, etc?)

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