This will be reused for __builtin_<op>_overflow, which gets checked in
SemaChecking.cpp instead and so can't currently use this.

Morello LLVM has downstream support for this, but it's both incomplete
(see https://git.morello-project.org/morello/llvm-project/-/issues/80)
and incorrect with regards to provenance (in that it takes a naive
type-based approach rather than considering the cheri_no_provenance
attribute, meaning it differs from the binary operators in provenance
semantics). This is a from-scratch implementation that aims to not have
the same shortcomings.