Fix grammar bugs around Declaration

- `Declaration` should contain `(Aggregate|Enum|Import)Declaration`
  Otherwise we cannot use them in statement scope via `DeclarationStatement`.

- Move out `AliasThis` from `Declaration`
  It cannot appear in statement scope.

- `StorageClass` and `Attribute` should be mostly same list
  Add missing `LinkageAttribute` and `AlignAttribute` in `StorageClass`.
  Add missing `pure`, `nothrow` in the list of `Attribute`.
  Add missing `ref` in both `Attribute` and `StorageClass`.

grammar.dd

@@ -1570,7 +1570,9 @@ $(H3 declaration)
 $(GRAMMAR
 $(GNAME Declaration):
     $(GLINK AliasDeclaration)
-    $(GLINK AliasThis)
+    $(GLINK AggregateDeclaration)
+    $(GLINK EnumDeclaration)
+    $(GLINK ImportDeclaration)
     $(GLINK Decl)
 )
 
@@ -1641,6 +1643,8 @@ $(GNAME StorageClasses):
     $(GLINK StorageClass) $(I StorageClasses)
 
 $(GNAME StorageClass):
+    $(GLINK LinkageAttribute)
+    $(GLINK AlignAttribute)
     $(D deprecated)
     $(D enum)
     $(D static)
@@ -1659,6 +1663,7 @@ $(GNAME StorageClass):
     $(GLINK Property)
     $(D pure)
     $(D nothrow)
+    $(D ref)
 )
 
 
@@ -2091,6 +2096,7 @@ $(GNAME Attribute):
     $(GLINK Property)
     $(D pure)
     $(D nothrow)
+    $(D ref)
 
 $(GNAME DeclarationBlock):
     $(GLINK DeclDef)
@@ -2219,9 +2225,6 @@ $(GNAME DeclDefs):
 
 $(GNAME DeclDef):
     $(GLINK AttributeSpecifier)
-    $(GLINK ImportDeclaration)
-    $(GLINK EnumDeclaration)
-    $(GLINK AggregateDeclaration)
     $(GLINK Declaration)
     $(GLINK Constructor)
     $(GLINK Destructor)
@@ -2230,6 +2233,7 @@ $(GNAME DeclDef):
     $(GLINK Deallocator)
     $(GLINK Invariant)
     $(GLINK UnitTest)
+    $(GLINK AliasThis)
     $(GLINK StaticConstructor)
     $(GLINK StaticDestructor)
     $(GLINK SharedStaticConstructor)