[ConstraintSystem] Add accessor to check whether target represents async let init

Author: xedin

include/swift/Sema/ConstraintSystem.h

@@ -1879,6 +1879,17 @@ class SolutionApplicationTarget {
         !expression.pattern->isImplicit();
   }
 
+  /// Check whether this is an initializaion for `async let` pattern.
+  bool isAsyncLetInitializer() const {
+    if (!(kind == Kind::expression &&
+          expression.contextualPurpose == CTP_Initialization))
+      return false;
+
+    if (auto *PBD = getInitializationPatternBindingDecl())
+      return PBD->isAsyncLet();
+    return false;
+  }
+
   /// Whether to bind the types of any variables within the pattern via
   /// one-way constraints.
   bool shouldBindPatternVarsOneWay() const {

lib/Sema/CSApply.cpp

@@ -8279,12 +8279,9 @@ static Optional<SolutionApplicationTarget> applySolutionToInitialization(
 
   // For an async let, wrap the initializer appropriately to make it a child
   // task.
-  if (auto patternBinding = target.getInitializationPatternBindingDecl()) {
-    if (patternBinding->isAsyncLet()) {
-      resultTarget.setExpr(
-          wrapAsyncLetInitializer(
-            cs, resultTarget.getAsExpr(), resultTarget.getDeclContext()));
-    }
+  if (target.isAsyncLetInitializer()) {
+    resultTarget.setExpr(wrapAsyncLetInitializer(
+        cs, resultTarget.getAsExpr(), resultTarget.getDeclContext()));
   }
 
   return resultTarget;