[StackProtector] Speed up RequiresStackProtector

Speed up the method RequiresStackProtector by checking the intrinsic
value of the call. The original code calls getName() that returns an
allocating std::string on each check. This change removes about 96072
std::string instances when compiling sqlite3.c; The function was
discovered with a Facebook-internal performance tool.

Differential Revision: https://reviews.llvm.org/D84620