[CS] Solve all conjunctions in source order
Previously we would only do source ordering for
ClosureExprs, but other conjunctions need to have
their source location taken into account too, in
order to make sure we don't try and type-check e.g
a TapExpr in a second closure before we type-check
the first closure.
Also while here, switch to `std::min_element`
instead of sorting, and treat invalid source
locations as incomparable.
rdar://113326835
