[analyzer] Add VforkChecker to find unsafe code in vforked process.

[analyzer] Add VforkChecker to find unsafe code in vforked process.

This checker looks for unsafe constructs in vforked process:
function calls (excluding whitelist), memory write and returns.
This was originally motivated by a vfork-related bug in xtables package.

Patch by Yury Gribov.

Differential revision: http://reviews.llvm.org/D14014

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@252285 91177308-0d34-0410-b5e6-96231b3b80d8