[UBSan] Add !nosanitize metadata to the code generated by UBSan.

This is used to mark the instructions emitted by Clang to implement
variety of UBSan checks. Generally, we don't want to instrument these
instructions with another sanitizers (like ASan).

Reviewed in http://reviews.llvm.org/D4544


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@213291 91177308-0d34-0410-b5e6-96231b3b80d8