[TypeChecker] ForEach: Don't diagnose unrelated errors if type doesn't conform to Sequence

If the type used in for-each loop doesn't conform to `Sequence`
let's not try to diagnose anything about its element since such
diagnostics would include unresolved types and actual problem
(missing conformance) would already be diagnosed.

Author: xedin

lib/Sema/TypeCheckConstraints.cpp

@@ -2857,6 +2857,11 @@ bool TypeChecker::typeCheckForEachBinding(DeclContext *dc, ForEachStmt *stmt) {
       ElementType = solution.simplifyType(ElementType);
       IteratorType = solution.simplifyType(IteratorType);
 
+      // If the type doesn't conform to Sequence we'll get its element type
+      // bound to `UnresolvedType` since fixes are allowed.
+      if (InitType->is<UnresolvedType>())
+        return nullptr;
+
       cs.cacheExprTypes(expr);
       Stmt->setSequence(expr);
       solution.setExprTypes(expr);

test/Constraints/generic_protocol_witness.swift

@@ -60,5 +60,4 @@ struct L<T>: Sequence {} // expected-error {{type 'L<T>' does not conform to pro
 
 func z(_ x: L<Int>) {
   for xx in x {}
-  // expected-warning@-1{{immutable value 'xx' was never used; consider replacing with '_' or removing it}}
 }

test/stmt/foreach.swift

@@ -14,7 +14,6 @@ struct BadContainer2 : Sequence { // expected-error{{type 'BadContainer2' does n
 
 func bad_containers_2(bc: BadContainer2) {
   for e in bc { }
-  // expected-warning@-1 {{immutable value 'e' was never used; consider replacing with '_' or removing it}}
 }
 
 struct BadContainer3 : Sequence { // expected-error{{type 'BadContainer3' does not conform to protocol 'Sequence'}}
@@ -23,7 +22,6 @@ struct BadContainer3 : Sequence { // expected-error{{type 'BadContainer3' does n
 
 func bad_containers_3(bc: BadContainer3) {
   for e in bc { }
-  // expected-warning@-1 {{immutable value 'e' was never used; consider replacing with '_' or removing it}}
 }
 
 struct BadIterator1 {}
@@ -35,7 +33,6 @@ struct BadContainer4 : Sequence { // expected-error{{type 'BadContainer4' does n
 
 func bad_containers_4(bc: BadContainer4) {
   for e in bc { }
-  // expected-warning@-1 {{immutable value 'e' was never used; consider replacing with '_' or removing it}}
 }
 
 // Pattern type-checking