Change CodeGenModule GlobalDeclMap to directly reference globals

Change CodeGenModule GlobalDeclMap to directly reference globals
instead of mapping the decl to a bitcast of the global to the correct
type.

 - GetAddrOf{Function,GlobalVar} introduce the bitcast on every use now.

 - This solves a problem where a dangling pointer could be introduced
   by the RAUW done when replacing a forward or tentative
   definition. See testcase for more details.

 - Fixes <rdar://problem/6108358>

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@54211 91177308-0d34-0410-b5e6-96231b3b80d8