AST: fix casting from Decl to GenericContext

Fixes `cast<GenericContext>(someDecl)`.
I don't know why this didn't show up as a crash so far. But slightly changing `Decl` results in a wrong type cast and a crash.

Author: eeckstein

include/swift/AST/Decl.h

@@ -1568,6 +1568,8 @@ class GenericContext : private _GenericContext, public DeclContext {
   /// Retrieve the position of any where clause for this context's
   /// generic parameters.
   SourceRange getGenericTrailingWhereClauseSourceRange() const;
+
+  static bool classof(const Decl *D);
 };
 static_assert(sizeof(_GenericContext) + sizeof(DeclContext) ==
               sizeof(GenericContext), "Please add fields to _GenericContext");
@@ -9589,6 +9591,16 @@ inline bool DeclContext::classof(const Decl *D) {
   }
 }
 
+inline bool GenericContext::classof(const Decl *D) {
+  switch (D->getKind()) { //
+  default: return false;
+#define DECL(ID, PARENT) // See previous line
+#define GENERIC_DECL(ID, PARENT) \
+  case DeclKind::ID: return true;
+#include "swift/AST/DeclNodes.def"
+  }
+}
+
 inline DeclContext *DeclContext::castDeclToDeclContext(const Decl *D) {
   // XXX -- ModuleDecl is not defined in Decl.h, but because DeclContexts
   // preface decls in memory, any DeclContext type will due.

include/swift/AST/DeclContext.h

@@ -1013,6 +1013,20 @@ namespace llvm {
     }
   };
 
+  template<class FromTy>
+  struct CastInfo<::swift::GenericContext, FromTy, std::enable_if_t<is_simple_type<FromTy>::value>>
+      : public CastIsPossible<::swift::GenericContext, FromTy>,
+        public DefaultDoCastIfPossible<::swift::GenericContext *, FromTy,
+                                       CastInfo<::swift::GenericContext, FromTy>> {
+    static inline ::swift::GenericContext *castFailed() { return nullptr; }
+
+    static inline ::swift::GenericContext *doCast(const FromTy &val) {
+      auto *genCtxt = val->getAsGenericContext();
+      assert(genCtxt);
+      return const_cast<::swift::GenericContext *>(genCtxt);
+    }
+  };
+
   template<class FromTy>
   struct CastInfo<::swift::IterableDeclContext, FromTy, std::enable_if_t<is_simple_type<FromTy>::value>>
       : public CastIsPossible<::swift::IterableDeclContext, FromTy>,