[Layering] Move GVMaterializer.h into the IR library where its
implementation already lived.

After this commit, the only IR-library headers in include/llvm/* are
ones related to the legacy pass infrastructure that I'm planning to
leave there until the new one is farther along.

The only other headers at the top level are linking and initialization
aids that aren't really libraries but just headers.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@203069 91177308-0d34-0410-b5e6-96231b3b80d8