[ubsan] Diagnose invalid uses of builtins (clang)

On some targets, passing zero to the clz() or ctz() builtins has undefined
behavior. I ran into this issue while debugging UB in __hash_table from libcxx:
the bug I was seeing manifested itself differently under -O0 vs -Os, due to a
UB call to clz() (see: libcxx/r304617).

This patch introduces a check which can detect UB calls to builtins.

llvm.org/PR26979

Differential Revision: https://reviews.llvm.org/D34590

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@309459 91177308-0d34-0410-b5e6-96231b3b80d8