[NFC][Concepts] Change the Source Range of Concepts ParamMatching

As came up in the discussion on
https://reviews.llvm.org/rG12cb1cb3720de8d164196010123ce1a8901d8122

We were asserting because the attempt to print a note found that our
source range for a immediately declared constraint (as a part of
Parameter Mapping Substitution) wasn't in order.

However, it doesn't really make sense to have the location of this be
the whole list of template arguments, as that would result in the range
being:
bool func(std::thing<char*> auto foo) {}
                     ^^^^^^^^^^^^^^^

Even if done correctly.  Instead, this patch makes the range be just
'foo' in this case (or a pointer right after 'auto' if unnamed).