Implement GNU asm-label extension support in CodeGen. This fixes

scimark2 on Darwin.

 - Added Sema support for asm-label on variables, which I forgot before.

 - Update CodeGen to use GlobalDeclMap to determine if static Decls
   require emission (instead of LLVM module name lookup). Important
   since the Decl name and the LLVM module name can differ.

 - <rdar://problem/6116729>

llvm-svn: 54388