[clang] Diagnose noreturn calls from a const or pure function (PR #206134)

[via cfe-commits]

Sirraide wrote:

@nikic Is calling a `noreturn` function from a `willreturn` function actually an issue in practice?

My two cents is that diagnosing this feels like it’ll cause a lot of false positives: `assert` and similar functions/macros often abort the program and end up calling `noreturn` functions inadvertently... which ought to be fine in a `const`/`pure` function—it doesn’t cause a problem if the assertion doesn’t fail, and conversely, if it _does_ fail, then that’s a bug in the program anyway.

Also, if this is semantically confusing, can we just remove `willreturn`? Does adding that make a difference in practice (it might, I’m not an optimiser guy candidly)?

https://github.com/llvm/llvm-project/pull/206134