[typechecker] remove protocol conformance diagnostic when type does not conform to Error when using is pattern in catch statement

Author: Suyash Srijan

lib/Sema/TypeCheckConstraints.cpp

@@ -4045,7 +4045,6 @@ CheckedCastKind TypeChecker::typeCheckCheckedCast(Type fromType,
         conformsToProtocol(toType, Context.getErrorDecl(), dc,
                            ConformanceCheckFlags::InExpression);
     if (fromType->isEqual(exceptionType) && !toTypeConformsToError) {
-      diagnose(diagLoc, diag::type_does_not_conform, toType, exceptionType);
       return failed();
     }
 

test/stmt/errors.swift

@@ -178,7 +178,7 @@ struct SR_6400_S_2: Error {}
 func sr_6400() throws {
   do {
     throw SR_6400_E.castError
-  } catch is SR_6400_S_1 { // expected-error {{type 'SR_6400_S_1' does not conform to protocol 'Error'}} expected-warning {{cast from 'Error' to unrelated type 'SR_6400_S_1' always fails}}
+  } catch is SR_6400_S_1 { // expected-warning {{cast from 'Error' to unrelated type 'SR_6400_S_1' always fails}}
     print("Caught error")
   }