[sanitizers] Disable target-specific lowering of string functions.

[sanitizers] Disable target-specific lowering of string functions.

CodeGen has hooks that allow targets to emit specialized code instead
of calls to memcmp, memchr, strcpy, stpcpy, strcmp, strlen, strnlen.
When ASan/MSan/TSan/ESan is in use, this sidesteps its interceptors, resulting
in uninstrumented memory accesses.  To avoid that, make these sanitizers
mark the calls as nobuiltin.

Differential Revision: http://reviews.llvm.org/D19781

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@273083 91177308-0d34-0410-b5e6-96231b3b80d8