[ASan] Add the memcmp_strict flag (1 by default) that controls the behavior of...

[ASan] Add the memcmp_strict flag (1 by default) that controls the behavior of accessibility checks in memcmp.
1: memcmp(p1, p2, n) always checks n bytes
0: memcmp checks up to n bytes depending on whether the memory contents differ.

llvm-svn: 176256