Types cannot contain Terms, so the Symbol representation uses
GenericTypeParameterTypes whose index refers to an array of
"substitution" Terms.
We can (ab)use the PrintOptions.AlternateTypeNames mechanism
to print those GenericTypeParamTypes as if they were Terms.
Previously we said that if P1 inherits from P2, then [P1:T] < [P2:T].
However, this didn't generalize to merged associated type symbols;
we always had [P1&P2:T] < [P3:T] even if P3 inherited from both P1
and P2.
This meant that the 'merge' operation did not preserve order, which
fired an assertion in the completion logic.
Fix this by generalizing the linear order to compare the 'support'
of protocols rather than the 'depth', where the 'support' is
defined as the size of the transitive closure of the set under
protocol inheritance.
Then, if you have something like
protocol P1 {}
protocol P2 {}
protocol P3 : P1, P2 {}
The support of 'P1 & P2' is 2, and the support of 'P3' is 3,
therefore [P3:T] < [P1&P2:T] in this example.
Fixes <rdar://problem/83768458>.
The canonical order on associated types compares the name before the
protocol, so for example T.[P2:A] < T.[P1:B]. Make sure the reduction
order does the same so that we correctly compute canonical types in
this case.
