[C API] Expose LLVMGetGlobalValueAddress and LLVMGetFunctionAddress.

Patch by Ramkumar Ramachandra <artagnon@gmail.com>

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@224720 91177308-0d34-0410-b5e6-96231b3b80d8