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>

llvm-svn: 54211