Change CodeGen of global decls to key off of the name (instead of

  having multiple bindings from all the possible decls which
  conceptually map to the same global).

 - This eliminates CodeGen depending on the LLVM module for name
   lookup.

 - This also eliminates the need for ReplaceMapValuesWith (hurrah).

 - This handles lookups for FunctionDecls correctly in the presence of
   aliases, this was previously broken.

 - WIP: Can still clean up & unify variable and function emission.

llvm-svn: 54382