[analyzer] Add a checker for mmap()s which are both writable and executable.

This is a security check which is disabled by default but will be enabled
whenever the user consciously enables the security package. If mmap()ed memory
is both writable and executable, it makes it easier for the attacker to execute
arbitrary code when contents of this memory are compromised. Some applications
require such mmap()s though, such as different sorts of JIT.

Patch by David Carlier!

Differential Revision: https://reviews.llvm.org/D42645

llvm-svn: 324166