[C] Add -Wjump-bypasses-init (#138009)
We already diagnose when a jump bypasses initialization in C++ because
such code is ill-formed there. However, we were not using this to
diagnose those same jumps in C.
-Wjump-bypasses-init is grouped under -Wc++-compat and diagnoses this
situation as a compatibility issue with C++. This diagnostic is off by
default.
The diagnostic could perhaps be enabled by default for C, but due to the
design of jump diagnostic handling, it not a trivial task. For now,
we'll add the diagnostic as off-by-default so we get incremental
improvement, but a follow-up could try to refactor jump diagnostics so
we can enable this by default in C and have it as a C++ compatibility
diagnostic as well.
