[ubsan] Skip overflow checks on safe arithmetic (fixes PR32874)

Currently, ubsan emits overflow checks for arithmetic that is known to
be safe at compile-time, e.g:

  1 + 1 => CheckedAdd(1, 1)

This leads to breakage when using the __builtin_prefetch intrinsic. LLVM
expects the arguments to @llvm.prefetch to be constant integers, and
when ubsan inserts unnecessary checks on the operands to the intrinsic,
this contract is broken, leading to verifier failures (see PR32874).

Instead of special-casing __builtin_prefetch for ubsan, this patch fixes
the underlying problem, i.e that clang currently emits unnecessary
overflow checks.

Testing: I ran the check-clang and check-ubsan targets with a stage2,
ubsan-enabled build of clang. I added a regression test for PR32874, and
some extra checking to make sure we don't regress runtime checking for
unsafe arithmetic. The existing ubsan-promoted-arithmetic.cpp test also
provides coverage for this change.

llvm-svn: 301988