refactor Parser::ParseStructDeclaration to return a vector of uninterpreted

declarators.  This allows the clients (C structs, objc classes, objc 
properties, [future] C++ classes) etc, to do custom processing before invoking
an action.

This has two benefits in the short term:
1) objc ivar processing should be split out of ActOnField into its own ActOn method.
2) the new objc ivar action can take visibility info directly, eliminating 
   AllVisibilities in ParseObjCClassInstanceVariables.
3) objc properties can pass their own special sauce down to sema as well.

llvm-svn: 49468