RequirementMachine: Drop GSB compatibility hack involving ErrorTypes

Another hack we can remove now that 'verify' mode skips checking if
there was a conflict.

Author: slavapestov

lib/AST/RequirementMachine/RequirementLowering.cpp

@@ -105,9 +105,6 @@ static void desugarSameTypeRequirement(Type lhs, Type rhs, SourceLoc loc,
     }
   } matcher(loc, result, errors);
 
-  if (lhs->hasError() || rhs->hasError())
-    return;
-
   (void) matcher.match(lhs, rhs);
 
   // If neither side is directly a type parameter, the type parameter

validation-test/compiler_crashers_2_fixed/0159-rdar40009245.swift

@@ -2,6 +2,7 @@
 
 protocol P {
     associatedtype A : P where A.X == Self
+    // expected-error@-1{{'X' is not a member type of type 'Self.A'}}
     associatedtype X : P where P.A == Self
     // expected-error@-1{{associated type 'A' can only be used with a concrete type or generic parameter base}}
 }