[cxx-interop] Suppress inaccessible-base warning from test (#85302)

Clang is helpfully telling us that one of the base types is
inaccessible, except we don't really care about that because we are only
using this type for testing other diagnostics. That warning is getting
caught by the newly comprehensive swiftc diagnostic verifier, so we
suppress it with a Clang pragma.

rdar://163875420

Author: j-hui

test/Interop/Cxx/foreign-reference/Inputs/inheritance.h

@@ -288,8 +288,11 @@ struct __attribute__((swift_attr("import_reference")))
 __attribute__((swift_attr("retain:retain1")))
 __attribute__((swift_attr("release:release1"))) B3 : B1 {};
 
+#pragma clang diagnostic push
+#pragma clang diagnostic ignored "-Winaccessible-base"
 struct D : B1, B2, B3 {}; // expected-warning {{unable to infer SWIFT_SHARED_REFERENCE for 'D', although one of its transitive base types is marked as SWIFT_SHARED_REFERENCE}}
 D *returnD() { return new D(); };
+#pragma clang diagnostic pop
 } // namespace MultipleInheritanceExample3
 
 void retain1(MultipleInheritanceExample3::B1 *v) {}