swift234

Sema: Clean up some code duplication when validating generic type signatures

Every call to validateGenericTypeSignature() had the same
boilerplate following; move the common logic into that
function.

As one might expect, each callsite had slight variants on
the same underlying logic -- this makes them consistent.

Also, this slightly widens the scope during which
GenericTypeDecl::isValidatingGenericSignature() returns
true.

Interesting, that change introduces a diagnostic in an
existing testcase where previously there was none:

protocol P {
  associatedtype T
}

struct S<A: P where A.T == S<A>> {}

While it looks like this generic signature was built
correctly, in fact I think we weren't computing
conformances for the substitution of 'A' in 'S<A>'.

After trying small variations on the above testcase,
I quickly ran into SILGen crashes, which the diagnostic
now prevents. A few interesting cases still crash.
See test/decl/protocol/req/recursion.swift for the
gory details.