refactor Parser::ParseStructDeclaration to return a vector of uninterpreted

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.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@49468 91177308-0d34-0410-b5e6-96231b3b80d8