[analyzer] Handle the M_ZERO and __GFP_ZERO flags in kernel mallocs.

[analyzer] Handle the M_ZERO and __GFP_ZERO flags in kernel mallocs.

Add M_ZERO awareness to malloc() static analysis in Clang for FreeBSD,
NetBSD, and OpenBSD in a similar fashion to O_CREAT for open(2).
These systems have a three-argument malloc() in the kernel where the
third argument contains flags; the M_ZERO flag will zero-initialize the
allocated buffer.

This should reduce the number of false positives when running static
analysis on BSD kernels.

Additionally, add kmalloc() (Linux kernel malloc()) and treat __GFP_ZERO
like M_ZERO on Linux.

Future work involves a better method of checking for named flags without
hardcoding values.

Patch by Conrad Meyer, with minor modifications by me.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@204832 91177308-0d34-0410-b5e6-96231b3b80d8