implement linking of globals to functions, in one direction

implement linking of globals to functions, in one direction
(replacing a function with a global).  This is needed when building
llvm itself with LTO on darwin, because of the EXPLICIT_SYMBOL hack
in lib/system/DynamicLibrary.cpp.

Implementation of linking the other way will need to wait for a
cleanup of LinkFunctionProtos.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@53546 91177308-0d34-0410-b5e6-96231b3b80d8