Addressed the issue in <rdar://problem/6479085>, where we failed to

rewrite @class declarations that showed up within linkage
specifications because those @class declarations never made it any
place where the rewriter could find them.

Moved all of the ObjC*Decl nodes over to ScopedDecls, so that they can
live in the appropriate top-level or transparent DeclContext near the
top level, e.g., TranslationUnitDecl or LinkageSpecDecl. Objective-C
declarations now show up in a traversal of the declarations in a
DeclContext (they didn't before!). This way, the rewriter finds all
Objective-C declarations within linkage specifications.

llvm-svn: 61966