Rework codegen emission of globals

 - No (intended) functionality change.

 - Primary purpose is to clearly separate (lazy) construction of
   globals that are a forward declaration or tentative definition from
   those that are the final definition.

 - Lazy construction is now encapsulated in
   GetAddrOf{Function,GlobalVar} while final definitions are
   constructed in EmitGlobal{Function,Var}Definition.
 
 - External interface for dealing with globals is now limited to
   EmitGlobal and GetAddrOf{Function,GlobalVar}.

 - Also updated helper functions dealing with statics, annotations,
   and ctors to be private.

llvm-svn: 54179