[msan] Check sigset_t and sigaction arguments.

Summary:
Check sigset_t arguments in ppoll, sig*wait*, sigprocmask
interceptors, and the entire "struct sigaction" in sigaction. This
can be done because sigemptyset/sigfullset are intercepted and
signal masks should be correctly marked as initialized.

Reviewers: vitalybuka

Subscribers: kubamracek, llvm-commits

Differential Revision: https://reviews.llvm.org/D37367

llvm-svn: 312576