Workaround an assertion failure during module build

After the change in https://reviews.llvm.org/D131858, clang cannot
bootstrap itself with modules due to assertion failure:
(lvaluePath->getType() == elemTy && "Unexpected type reference!")

Workaround the assertion by converting some of the includes into forward
declares.

Reviewed By: arphaman

Differential Revision: https://reviews.llvm.org/D139956