Importing `type_traits` from libstdc++ currently causes a crash on Linux:
```
swift-ide-test: tools/clang/include/clang/AST/TypeNodes.inc:33: clang::TypeInfo clang::ASTContext::getTypeInfoImpl(const clang::Type *) const: Assertion `!T->isDependentType() && "should not see dependent types here"' failed.
PLEASE submit a bug report to https://bugs.llvm.org/ and include the crash backtrace.
Stack dump:
0. Program arguments: /home/egorzh/Builds/swift/swift/bin/swift-ide-test -print-module -module-to-print=std -source-filename=x -enable-cxx-interop
1. /usr/lib/gcc/x86_64-linux-gnu/9/../../../../include/c++/9/type_traits:1110:10: importing 'std::__do_is_implicitly_default_constructible_impl'
2. /usr/lib/gcc/x86_64-linux-gnu/9/../../../../include/c++/9/type_traits:1116:22: importing 'std::__do_is_implicitly_default_constructible_impl::__test'
```
This change fixes the crash by bailing on such functions.
This is important because the specialized template will be imported as `inout int` (for example) and we must make sure to match the signature.
Updates tests based on this and fixes a few tests that started failing after daceecfc75.
C++ namespaces are module-independent, but enums are owned by their module's in Swift. So, to prevent declaring two enums with the same name, this patch implements a new approach to namespaces: enums with extensions.
Here's an example:
```
// Module A
namespace N { void test1(); }
// Module B
namespace N { void test2(); }
// __ObjC module
enum N { }
// Swift module A
extension N { func test1() }
// Swift module B
extension N { func test1() }
```
Thanks to @gribozavr for the great idea.
