[Strict memory safety] Fix "unsafe" checking for the for..in loop

The `$generator` variable we create for the async for..in loop is
`nonisolated(unsafe)`, so ensure that we generate an `unsafe`
expression when we use it. This uncovered some inconsistencies in how
we do `unsafe` checking for for..in loops, so fix those.

Fixes rdar://154775389.

test/Unsafe/safe.swift

@@ -98,6 +98,8 @@ func testUnsafeAsSequenceForEach() {
   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
+
+  for unsafe _ in [1, 2, 3] { } // expected-warning{{no unsafe operations occur within 'unsafe' for-in loop}}
 }
 
 func testForInUnsafeAmbiguity(_ integers: [Int]) {

test/Unsafe/unsafe_concurrency.swift

@@ -55,3 +55,10 @@ open class SyntaxVisitor {
 open class SyntaxAnyVisitor: SyntaxVisitor {
   override open func visit(_ token: TokenSyntax) { }
 }
+
+@available(SwiftStdlib 5.1, *)
+func testMemorySafetyWithForLoop() async {
+  let (stream, continuation) = AsyncStream<Int>.makeStream()
+  for await _ in stream {}
+  _ = continuation
+}