[PatternMatch] Implement matching code for LibFunc

Summary: Provides m_LibFunc pattern that can be used to match LibFuncs.

Reviewers: spatel, hfinkel, efriedma, lebedev.ri

Reviewed By: lebedev.ri

Subscribers: lebedev.ri, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D42047

llvm-svn: 366069