[cxx-interop] Members of private base classes should not be exposed

[egorzhdan]

Trying to use members of C++ private base classes from Swift causes an assertion failure in ClangImporter:

<build dir>/swift/lib/swift/macosx/arm64/libcxxshim.h:2:66: error: cannot cast 'A' to its private base class 'B'
To __swift_interopStaticCast(From from) { return static_cast<To>(from); }
                                                                 ^

Assertion failed: (Val && "isa<> used on a null pointer"), function doit, file Casting.h, line 104.
Please submit a bug report (https://swift.org/contributing/#reporting-bugs) and include the crash backtrace.
Stack dump:
...
8  swift-frontend           llvm::isa_impl_cl<swift::FuncDecl, swift::Decl const*>::doit(swift::Decl const*) + 100
9  swift-frontend           llvm::isa_impl_wrap<swift::FuncDecl, swift::Decl const*, swift::Decl const*>::doit(swift::Decl const* const&) + 28
10 swift-frontend           llvm::isa_impl_wrap<swift::FuncDecl, swift::Decl* const, swift::Decl const*>::doit(swift::Decl* const&) + 40
11 swift-frontend           bool llvm::isa<swift::FuncDecl, swift::Decl*>(swift::Decl* const&) + 24
12 swift-frontend           llvm::cast_retty<swift::FuncDecl, swift::Decl*>::ret_type llvm::cast<swift::FuncDecl, swift::Decl>(swift::Decl*) + 28
13 swift-frontend           getInteropStaticCastDeclRefExpr(swift::ASTContext&, clang::Module const*, swift::Type, swift::Type) + 768
14 swift-frontend           synthesizeBaseClassMethodBody(swift::AbstractFunctionDecl*, void*)::$_30::operator()() const + 396
...

Such members should not be exposed.

rdar://103871000

[egorzhdan]

@swift-ci please smoke test

[egorzhdan]

@swift-ci please smoke test macOS

[zoecarver]

Awesome! Thanks, Egor!