[Sema] Add missing null check for Type

`getType` here can return a null type if the queried expression isn't
part of the solution, which can currently happen for code completion.
I'm working on a more principled fix for this, but until then this is
a low-risk fix that will unblock the stress tester and can be
cherry-picked to 6.2.

rdar://149759542

test/SourceKit/CodeComplete/issue-80985.swift

@@ -0,0 +1,13 @@
+// https://github.com/swiftlang/swift/issues/80985
+struct S<T> {
+  func foo<U>(_ fn: (T) -> U) -> S<U> { fatalError() }
+}
+
+func foo(xs: S<(Int, Int)>) {
+  _ = {
+    let y = xs
+      .foo{ $1 }
+      .foo{ $0 }
+    // RUN: %sourcekitd-test -req=complete -pos=%(line-1):11 %s -- %s
+  }
+}