[Strict memory safety] Show issues with unsafe constructs in the for..in loop

We weren't showing the unsafe uses when we determine that a for..in
loop is unsafe. Do so, which generally means complaining about `next()`
being unsafe. Fixes rdar://151237127

lib/Sema/TypeCheckEffects.cpp

@@ -4597,6 +4597,10 @@ private:
         auto insertionLoc = S->getPattern()->getStartLoc();
         Ctx.Diags.diagnose(S->getForLoc(), diag::for_unsafe_without_unsafe)
           .fixItInsert(insertionLoc, "unsafe ");
+
+        for (const auto &unsafeUse : classification.getUnsafeUses()) {
+          diagnoseUnsafeUse(unsafeUse);
+        }
       }
     } else if (S->getUnsafeLoc().isValid()) {
       // Extraneous "unsafe" on the sequence.

test/Unsafe/safe.swift

@@ -89,10 +89,12 @@ struct UnsafeAsSequence: @unsafe Sequence, @unsafe IteratorProtocol {
 func testUnsafeAsSequenceForEach() {
   let uas = UnsafeAsSequence()
 
+  // expected-note@+2{{reference to unsafe instance method 'next()'}}
   // expected-warning@+1{{expression uses unsafe constructs but is not marked with 'unsafe'}}{{12-12=unsafe }}
   for _ in uas { } // expected-note{{conformance}}
   // expected-warning@-1{{for-in loop uses unsafe constructs but is not marked with 'unsafe'}}{{documentation-file=strict-memory-safety}}{{7-7=unsafe }}
 
+  // expected-note@+1{{reference to unsafe instance method 'next()'}}
   for _ in unsafe uas { } // expected-warning{{for-in loop uses unsafe constructs but is not marked with 'unsafe'}}{{documentation-file=strict-memory-safety}}{{7-7=unsafe }}
 
   for unsafe _ in unsafe uas { } // okay
@@ -118,6 +120,7 @@ struct SequenceWithUnsafeIterator: Sequence {
 func testUnsafeIteratorForEach() {
   let swui = SequenceWithUnsafeIterator()
 
+  // expected-note@+1{{reference to unsafe instance method 'next()'}}
   for _ in swui { } // expected-warning{{for-in loop uses unsafe constructs but is not marked with 'unsafe'}}{{7-7=unsafe }}
   for unsafe _ in swui { } // okay, it's only the iterator that's unsafe
 }

test/Unsafe/unsafe-suppression.swift

@@ -144,9 +144,12 @@ struct UnsafeSequence: @unsafe IteratorProtocol, @unsafe Sequence {
 }
 
 func forEachLoop(us: UnsafeSequence) {
+  // expected-note@+1{{reference to unsafe instance method 'next()'}}
   for _ in us { } // expected-warning{{expression uses unsafe constructs but is not marked with 'unsafe'}}{{documentation-file=strict-memory-safety}}{{12-12=unsafe }}
   // expected-note@-1{{'@unsafe' conformance of 'UnsafeSequence' to protocol 'Sequence' involves unsafe code}}
   // expected-warning@-2{{for-in loop uses unsafe constructs but is not marked with 'unsafe'}}{{documentation-file=strict-memory-safety}}
+
+  // expected-note@+1{{reference to unsafe instance method 'next()'}}
   for _ in unsafe us { }
   // expected-warning@-1{{for-in loop uses unsafe constructs but is not marked with 'unsafe'}}{{documentation-file=strict-memory-safety}}
 }