Bail out earlier when we try to recursively validate a generic signature.

Fixes one of the crashers I recently regressed.

Author: DougGregor

lib/Sema/TypeCheckType.cpp

@@ -526,7 +526,7 @@ Type TypeChecker::applyUnboundGenericArguments(
     auto genericSig = unbound->getDecl()->getGenericSignature();
     if (unbound->getDecl()->IsValidatingGenericSignature()) {
       diagnose(loc, diag::recursive_requirement_reference);
-      return BGT;
+      return nullptr;
     }
     assert(genericSig != nullptr);
     if (checkGenericArguments(dc, loc, noteLoc, unbound, genericSig,

test/Generics/generic_types.swift

@@ -312,9 +312,7 @@ var y: X5<X4, Int> // expected-error{{'X5' requires the types 'AssocP' (aka 'Int
 
 // Recursive generic signature validation.
 class Top {}
-class Bottom<T : Bottom<Top>> {} // expected-error 2{{type may not reference itself as a requirement}}
-// expected-error@-1{{Bottom' requires that 'Top' inherit from 'Bottom<Top>'}}
-// expected-note@-2{{requirement specified as 'T' : 'Bottom<Top>' [with T = Top]}}
+class Bottom<T : Bottom<Top>> {} // expected-error {{type may not reference itself as a requirement}}
 
 class X6<T> {
   let d: D<T>

validation-test/compiler_crashers_fixed/26403-llvm-densemapbase-llvm-smalldensemap-std-pair-llvm-arrayref-std-pair-swift-identifier.swift

@@ -8,5 +8,3 @@
 // RUN: not %target-swift-frontend %s -parse
 class B<T where B:d
 class B<T>:B<T>
-
-// REQUIRES: disabled