[Concurrency] Extract the check for async context to the namespace

It's useful not only inside of isolation checking but in the
type-checker as well.

Author: xedin

lib/Sema/TypeCheckConcurrency.cpp

@@ -1796,20 +1796,7 @@ namespace {
     }
 
     bool isInAsynchronousContext() const {
-      auto dc = getDeclContext();
-      if (auto func = dyn_cast<AbstractFunctionDecl>(dc)) {
-        return func->isAsyncContext();
-      }
-
-      if (auto closure = dyn_cast<AbstractClosureExpr>(dc)) {
-        if (auto type = closure->getType()) {
-          if (auto fnType = type->getAs<AnyFunctionType>()) {
-            return fnType->isAsync();
-          }
-        }
-      }
-
-      return false;
+      return isAsynchronousContext(getDeclContext());
     }
 
     enum class AsyncMarkingResult {
@@ -4282,3 +4269,19 @@ AnyFunctionType *swift::adjustFunctionTypeForConcurrency(
 bool swift::completionContextUsesConcurrencyFeatures(const DeclContext *dc) {
   return contextUsesConcurrencyFeatures(dc);
 }
+
+bool swift::isAsynchronousContext(const DeclContext *dc) {
+  if (auto func = dyn_cast<AbstractFunctionDecl>(dc)) {
+    return func->isAsyncContext();
+  }
+
+  if (auto closure = dyn_cast<AbstractClosureExpr>(dc)) {
+    if (auto type = closure->getType()) {
+      if (auto fnType = type->getAs<AnyFunctionType>()) {
+        return fnType->isAsync();
+      }
+    }
+  }
+
+  return false;
+}

lib/Sema/TypeCheckConcurrency.h

@@ -328,6 +328,11 @@ bool isDispatchQueueOperationName(StringRef name);
 /// \returns true if an error occurred.
 bool checkSendableConformance(
     ProtocolConformance *conformance, SendableCheck check);
+
+/// Check whether the given declaration context is asynchronous e.g.
+/// function or a closure declaration marked as `async`.
+bool isAsynchronousContext(const DeclContext *dc);
+
 } // end namespace swift
 
 #endif /* SWIFT_SEMA_TYPECHECKCONCURRENCY_H */