[Sema] Add skeleton implementation of @runtimeMetadata attribute

Use of the attribute is gated on `RuntimeDiscoverableAttrs` flag.

Author: xedin

include/swift/AST/DiagnosticsSema.def

@@ -6756,6 +6756,14 @@ ERROR(moveonly_cannot_conform_to_protocol, none,
 ERROR(moveonly_cannot_conform_to_protocol_with_name, none,
      "move-only %0 %1 cannot conform to protocol %2",
      (DescriptiveDeclKind, DeclName, DeclName))
+
+//------------------------------------------------------------------------------
+// MARK: Runtime discoverable attributes (@runtimeMetadata)
+//------------------------------------------------------------------------------
+
+ERROR(runtime_discoverable_attrs_are_experimental,none,
+      "runtime discoverable attributes are an experimental feature", ())
+
 #define UNDEFINE_DIAGNOSTIC_MACROS
 #include "DefineDiagnosticMacros.h"
 

lib/Sema/TypeCheckAttr.cpp

@@ -336,6 +336,8 @@ class AttributeChecker : public AttributeVisitor<AttributeChecker> {
   void visitKnownToBeLocalAttr(KnownToBeLocalAttr *attr);
 
   void visitSendableAttr(SendableAttr *attr);
+
+  void visitRuntimeMetadataAttr(RuntimeMetadataAttr *attr);
 };
 
 } // end anonymous namespace
@@ -6934,6 +6936,15 @@ void AttributeChecker::visitCompilerInitializedAttr(
   }
 }
 
+void AttributeChecker::visitRuntimeMetadataAttr(RuntimeMetadataAttr *attr) {
+  if (!Ctx.LangOpts.hasFeature(Feature::RuntimeDiscoverableAttrs)) {
+    diagnose(attr->getLocation(),
+             diag::runtime_discoverable_attrs_are_experimental);
+    attr->setInvalid();
+    return;
+  }
+}
+
 namespace {
 
 class ClosureAttributeChecker

lib/Sema/TypeCheckDeclOverride.cpp

@@ -1623,6 +1623,8 @@ namespace  {
 
     UNINTERESTING_ATTR(EagerMove)
     UNINTERESTING_ATTR(NoEagerMove)
+
+    UNINTERESTING_ATTR(RuntimeMetadata)
 #undef UNINTERESTING_ATTR
 
     void visitAvailableAttr(AvailableAttr *attr) {