[ObjC++] Don't enter a C++ declarator scope when the current context is

an Objective-C declaration

This commit ensures that Sema won't enter a C++ declarator scope when the
current context is an Objective-C declaration. This prevents an assertion
failure in EnterDeclaratorContext that's used to ensure that current context
will be restored correctly after exiting the declarator context.

rdar://20560175

Differential Revision: https://reviews.llvm.org/D26922

llvm-svn: 288893