Clang suppresses template specialization if a fatal error has been
reported in order to reduce the risk of a cascade of secondary error
diagnostics.

However, DXC DXASSERTs if template specialization fails - even if that
is due to an unrelated fatal error - which has the unintended result of
hiding the fatal error and hence providing no indication of what the
problem is.

The DXASSERT conditions have been amended so they are no longer raised
if a fatal error has been registered.

Amend the fix for DXASSERTS in template specialization by incorporating
changes from review comments.