[cxx-interop] Revert changes to ClangTypeConverter::visitMetatypeType.

The old implementation should be sufficient. The C++ constructors we
import should get their types from the imported decl, not a double
conversion to a SILFunction type and then back again.

Author: zoecarver

lib/AST/ClangTypeConverter.cpp

@@ -461,19 +461,7 @@ clang::QualType ClangTypeConverter::visitProtocolType(ProtocolType *type) {
 // Metatypes can be converted to Class when they are metatypes for concrete
 // classes. https://github.com/apple/swift/pull/27479#discussion_r344418131
 clang::QualType ClangTypeConverter::visitMetatypeType(MetatypeType *type) {
-  assert(type->hasRepresentation() &&
-         "metatype should have been assigned a representation");
-  switch (type->getRepresentation()) {
-  case MetatypeRepresentation::Thin:
-    return ClangASTContext.VoidTy;
-
-  case MetatypeRepresentation::Thick:
-    llvm_unreachable("thick metatypes don't have a corresponding Clang type");
-
-  case MetatypeRepresentation::ObjC:
-    return getClangMetatypeType(ClangASTContext);
-  }
-  llvm_unreachable("bad representation");
+  return getClangMetatypeType(ClangASTContext);
 }
 
 // TODO: [stronger-checking-in-clang-type-conversion]