[Frontend] Hide @execution attribute behind an experimental feature ExecutionAttribute

Since the proposal has not been approved yet we cannot expose
`@execution` attribute.

lib/AST/FeatureSet.cpp

@@ -416,6 +416,45 @@ static bool usesFeatureBuiltinEmplaceTypedThrows(Decl *decl) {
   return false;
 }
 
+static bool usesFeatureExecutionAttribute(Decl *decl) {
+  if (decl->getAttrs().hasAttribute<ExecutionAttr>())
+    return true;
+
+  auto VD = dyn_cast<ValueDecl>(decl);
+  if (!VD)
+    return false;
+
+  auto hasExecutionAttr = [](TypeRepr *R) {
+    if (!R)
+      return false;
+
+    return R->findIf([](TypeRepr *repr) {
+      if (auto *AT = dyn_cast<AttributedTypeRepr>(repr)) {
+        return llvm::any_of(AT->getAttrs(), [](TypeOrCustomAttr attr) {
+          if (auto *TA = attr.dyn_cast<TypeAttribute *>()) {
+            return isa<ExecutionTypeAttr>(TA);
+          }
+          return false;
+        });
+      }
+      return false;
+    });
+  };
+
+  // Check if any parameters that have `@execution` attribute.
+  if (auto *PL = getParameterList(VD)) {
+    for (auto *P : *PL) {
+      if (hasExecutionAttr(P->getTypeRepr()))
+        return true;
+    }
+  }
+
+  if (hasExecutionAttr(VD->getResultTypeRepr()))
+    return true;
+
+  return false;
+}
+
 // ----------------------------------------------------------------------------
 // MARK: - FeatureSet
 // ----------------------------------------------------------------------------