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

This is a security check that warns when both PROT_WRITE and PROT_EXEC are
set during mmap(). 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.

Re-applied after a revert in r324167.

Temporarily stays in the alpha package because it needs a better way of
determining macro values that are not immediately available in the AST.

Patch by David Carlier!

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

llvm-svn: 326405