[ASTScope] Eliminate getDeclAttributeIfAny().

We only used this in one place, and then immediately filtered down
to a `CustomAttr`. Just use `dyn_cast` in the obvious way and remove
this function.

Author: DougGregor

include/swift/AST/ASTScope.h

@@ -237,7 +237,6 @@ class ASTScopeImpl : public ASTAllocated<ASTScopeImpl> {
   NullablePtr<Decl> getDeclIfAny() const;
   NullablePtr<Stmt> getStmtIfAny() const;
   NullablePtr<Expr> getExprIfAny() const;
-  NullablePtr<DeclAttribute> getDeclAttributeIfAny() const;
 
 #pragma mark - debugging and printing
 

lib/AST/ASTScope.cpp

@@ -170,22 +170,6 @@ NullablePtr<Expr> ASTScopeImpl::getExprIfAny() const {
   }
 }
 
-NullablePtr<DeclAttribute> ASTScopeImpl::getDeclAttributeIfAny() const {
-  switch (getKind()) {
-    // Statement scope nodes extract the statement directly.
-#define DECL_ATTRIBUTE_SCOPE_NODE(Name) \
-    case ScopeKind::Name: return cast<Name##Scope>(this)->getDeclAttr();
-#define SCOPE_NODE(Name)
-#include "swift/AST/ASTScopeNodes.def"
-
-    // Everything else returns nullptr.
-#define DECL_ATTRIBUTE_SCOPE_NODE(Name)
-#define SCOPE_NODE(Name) case ScopeKind::Name:
-#include "swift/AST/ASTScopeNodes.def"
-      return nullptr;
-  }
-}
-
 SourceManager &ASTScopeImpl::getSourceManager() const {
   return getASTContext().SourceMgr;
 }

lib/AST/ASTScopeLookup.cpp

@@ -649,10 +649,10 @@ void ASTScopeImpl::lookupEnclosingMacroScope(
         return;
     }
 
-    auto *attr = scope->getDeclAttributeIfAny().getPtrOrNull();
-    auto *potentialAttached = dyn_cast_or_null<CustomAttr>(attr);
-    if (potentialAttached && consume(potentialAttached))
-      return;
+    if (auto customAttrScope = dyn_cast<CustomAttributeScope>(scope)) {
+      if (consume(customAttrScope->attr))
+        return;
+    }
 
     // If we've reached a source file scope, we can't be inside of
     // a macro argument. Either this is a top-level source file, or