When constructing SwiftLookupTable at clang module creation time, calls
to `clang::Sema::LookupName` will fail in C++ mode (including ObjC++).
The specific reason is that `clangSema.TUScope` (which we are passing in
as a Scope) is `nullptr` as we have no active Parser. The C++ path in
`clang::Sema::LookupName` is set to fail early and hard when the Scope
passed in is nullptr. In most, if not all, calls to `LookupName`, we
care about ObjC symbols and not C++ names. For example, the motivation
behind this issue is that ObjC protocols are not being renamed when
there is an ObjC class with the same name, leading to compilation
failures.
