Fix a failed assertion on an invalid typename requirement

The parsing code for a typename requirement currently asserts when
given something which is not a valid type-requirement
(http://eel.is/c++draft/expr.prim.req.type#nt:type-requirement). This
removes the assertion to continue on to the proper diagnostic.

This resolves PR53057.

Note that in that PR, it is using _BitInt(N) as a dependent type name.
This patch does not attempt to support that as it is not clear that is
a valid type requirement (it does not match the grammar production for
one). The workaround in the PR, however, is definitely valid and works
as expected.