[CSDiagnostics] Fix trailing closure ambiguity note to not expect that anchor is expression

`TrailingClosureAmbiguityFailure::diagnoseAsNote()` is used by
`diagnoseAmbiguity` opportunistically, which means that anchor
could be a pattern or a statement condition element.

Author: xedin

lib/Sema/CSDiagnostics.cpp

@@ -1746,7 +1746,12 @@ bool TypeChecker::diagnoseSelfAssignment(const Expr *expr) {
 }
 
 bool TrailingClosureAmbiguityFailure::diagnoseAsNote() {
-  auto *anchor = castToExpr(getAnchor());
+  auto *anchor = getAsExpr(getAnchor());
+  // This diagnostic is used opportunistically in `diagnoseAmbiguity`,
+  // which means it cannot assume that anchor is always an expression.
+  if (!anchor)
+    return false;
+
   const auto *expr = findParentExpr(anchor);
   auto *callExpr = dyn_cast_or_null<CallExpr>(expr);
   if (!callExpr)

test/expr/closure/multi_statement.swift

@@ -234,6 +234,20 @@ func test_local_function_capturing_vars() {
   }
 }
 
+func test_pattern_ambiguity_doesnot_crash_compiler() {
+  enum E {
+  case hello(result: Int) // expected-note {{found this candidate}}
+  case hello(status: Int) // expected-note {{found this candidate}}
+  }
+
+  let _: (E) -> Void = {
+    switch $0 {
+    case let E.hello(x): print(x) // expected-error {{ambiguous use of 'hello'}}
+    default: break
+    }
+  }
+}
+
 func test_taps_type_checked_with_correct_decl_context() {
   struct Path {
     func contains<T>(_: T) -> Bool where T: StringProtocol { return false }