Add static checking to ensure every DeclAttribute subclass has a properly-typed clone()

Code review identified some incorrect UNIMPLEMENTED_CLONEs in DeclAttribute (thank you
Hamish and Rintaro). Fix those, and make sure this can't happen again by checking the type
signatures of clone() in every DeclAttribute subclass.

Author: DougGregor

include/swift/AST/Attr.h

@@ -1164,7 +1164,7 @@ class TypeEraserAttr final : public DeclAttribute {
     return DA->getKind() == DeclAttrKind::TypeEraser;
   }
 
-  UNIMPLEMENTED_CLONE(DynamicReplacementAttr)
+  UNIMPLEMENTED_CLONE(TypeEraserAttr)
 };
 
 /// Represents any sort of access control modifier.
@@ -2814,7 +2814,7 @@ class RawLayoutAttr final : public DeclAttribute {
     return DA->getKind() == DeclAttrKind::RawLayout;
   }
 
-  UNIMPLEMENTED_CLONE(MacroRoleAttr)
+  UNIMPLEMENTED_CLONE(RawLayoutAttr)
 };
 
 class LifetimeAttr final : public DeclAttribute {

lib/AST/Attr.cpp

@@ -353,7 +353,16 @@ bool DeclAttribute::canAttributeAppearOnDeclKind(DeclAttrKind DAK, DeclKind DK)
   llvm_unreachable("bad DeclKind");
 }
 
+// Ensure that every DeclAttribute subclass implements its own CloneAttr.
+static void checkDeclAttributeClones() {
+#define DECL_ATTR(_,CLASS,...) \
+  CLASS##Attr *(CLASS##Attr::*ptr##CLASS)(ASTContext &) const = &CLASS##Attr::clone; \
+  (void)ptr##CLASS;
+#include "swift/AST/DeclAttr.def"
+}
+
 DeclAttribute *DeclAttribute::clone(ASTContext &ctx) const {
+  (void)checkDeclAttributeClones;
   switch (getKind()) {
 #define DECL_ATTR(_,CLASS, ...) \
   case DeclAttrKind::CLASS: return static_cast<const CLASS##Attr *>(this)->clone(ctx);