[LLVM-C] Add an accessor for the "value type" of a global

Summary: Before this, there was no reasonable way to retrieve the type of a global value (most notably, a function) that was created with  the C API.

Reviewers: whitequark, deadalnix

Reviewed By: whitequark

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D52659

llvm-svn: 343363