Disable -Wsometimes-uninitialized warnings in the 9.2 branch.

Disable -Wsometimes-uninitialized warnings in the 9.2 branch.

In 9.2, we didn't yet teach the compiler that elog(ERROR) doesn't
return, which means that the compiler's opinion about whether
variable dereferences are reachable isn't worth much.  Back-patching
the commits that fixed that seems unduly invasive, so instead silence
related complaints with a big hammer.

-Wsometimes-uninitialized is a clang-ism; gcc seems to spell
this warning option differently.  However, I do not observe
any such compiler warnings in 9.2 with gcc 11.2.1, so for now
we needn't do anything for gcc.

Discussion: https://postgr.es/m/d0316012-ece7-7b7e-2d36-9c38cb77cb3b@enterprisedb.com