[-Wunsafe-buffer-usage] Handle unevaluated contexts that contain unsafe buffer usages

This patch handles unevaluated contexts to ensure no warnings are produced by the machinery
for buffer access made within an unevaluated contexts. However, such accesses must be
considered by a FixableGadget and produce the necessary fixits.

Reviewed by: NoQ, ziqingluo-90, jkorous

Differential revision: https://reviews.llvm.org/D144905