Added "InitializeTU" to ASTConsumer. This is used by Sema::ParseAST to pass a

TranslationUnit object instead of an ASTContext. By default it calls
Initialize(ASTConstext& Context) (to match with the current interface used by
most ASTConsumers).

Modified the ObjC-Rewriter to use InitializeTU, and to tell the TranslationUnit
to not free its Decls.  This is a workaround for: <rdar://problem/5966749>

llvm-svn: 51825