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

[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

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@288893 91177308-0d34-0410-b5e6-96231b3b80d8