[clang] fix concepts crash on substitution failure during normalization

When substitution failed on the first constrained template argument (but
only the first), we would assert / crash. Checking for failure was only
being performed from the second constraint on.

This changes it so the checking is performed in that case,
and the code is also now simplified a little bit to hopefully
avoid this confusion.

Signed-off-by: Matheus Izvekov <mizvekov@gmail.com>

Reviewed By: rsmith

Differential Revision: https://reviews.llvm.org/D106907