[C] Make (c ? e1 : e2) noreturn only if both e1 and e2 are noreturn

In C mode, if e1 has __attribute__((noreturn)) but e2 doesn't, `(c ? e1 : e2)`
is incorrectly noreturn and Clang codegen produces `unreachable`
which may lead to miscompiles (see [1] `gawk/support/dfa.c`).
This problem has been known since
8c6b56f3 (2010) or earlier.

Fix this by making the result type noreturn only if both e1 and e2 are
noreturn, matching GCC.

`_Noreturn` and `[[noreturn]]` do not have the aforementioned problem.

Fix https://github.com/llvm/llvm-project/issues/59792 [1]

Reviewed By: aaron.ballman

Differential Revision: https://reviews.llvm.org/D140868