[ConstraintSystem] Update closure inference to support @execution(...) attribute

- Update `ClosureEffectsRequest` to make sure that closure is
marked as `async` when `@execution(...)` attribute is present.

- Update `inferClosureType` to set isolation based on an explicit
`@execution(...)` attribute attached to a closure.

Author: xedin

lib/Sema/CSGen.cpp

@@ -2542,6 +2542,16 @@ namespace {
             return FunctionTypeIsolation::forGlobalActor(actorType);
         }
 
+        if (auto *execution =
+                closure->getAttrs().getAttribute<ExecutionAttr>()) {
+          switch (execution->getBehavior()) {
+          case ExecutionKind::Caller:
+            return FunctionTypeIsolation::forNonIsolatedCaller();
+          case ExecutionKind::Concurrent:
+            return FunctionTypeIsolation::forNonIsolated();
+          }
+        }
+
         return FunctionTypeIsolation::forNonIsolated();
       }();
       extInfo = extInfo.withIsolation(isolation);

lib/Sema/ConstraintSystem.cpp

@@ -1400,6 +1400,12 @@ FunctionType::ExtInfo ClosureEffectsRequest::evaluate(
   bool throws = expr->getThrowsLoc().isValid();
   bool async = expr->getAsyncLoc().isValid();
   bool sendable = expr->getAttrs().hasAttribute<SendableAttr>();
+
+  // `@execution(...)` attribute is only valid on asynchronous function types.
+  if (expr->getAttrs().hasAttribute<ExecutionAttr>()) {
+    async = true;
+  }
+
   if (throws || async) {
     return ASTExtInfoBuilder()
       .withThrows(throws, /*FIXME:*/Type())

test/Concurrency/attr_execution_conversions.swift

@@ -82,6 +82,17 @@ do {
   // expected-error@-1 {{cannot convert value of type '(@execution(caller) () async -> ()).Type' to expected argument type '(@isolated(any) () async -> Void).Type'}}
 }
 
+do {
+  let _: () -> Void = { @execution(concurrent) in
+    // expected-error@-1 {{invalid conversion from 'async' function of type '() async -> Void' to synchronous function type '() -> Void'}}
+  }
+
+  func test(_: () -> Void) {}
+
+  test { @execution(caller) in
+    // expected-error@-1 {{cannot pass function of type '@execution(caller) () async -> ()' to parameter expecting synchronous function type}}
+  }
+}
 
 // Converting to `@execution(caller)` function
 class NonSendable {}