Use 'waitpid' instead of 'wait'. Basing Program::Wait() on 'wait()' prevents it...

Use 'waitpid' instead of 'wait'.  Basing Program::Wait() on 'wait()' prevents it being correct within a multithreaded context.

This address: PR 5277 (Program::Wait is unsafe to call from multiple threads).

Note: If waitpid() turns out to be non-portable, we can add more autoconf magic, or look into
another solution.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@84903 91177308-0d34-0410-b5e6-96231b3b80d8