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