[ASTPrint] Refactor CustomAttr printing (NFC)

This is a pure refactor to enable using PrintAST in the next commit.

Author: hnrklssn

include/swift/AST/Attr.h

@@ -2344,6 +2344,8 @@ class CustomAttr final : public DeclAttribute {
 
   bool isEquivalent(const CustomAttr *other, Decl *attachedTo) const;
 
+  void printCustomAttr(ASTPrinter &Printer, const PrintOptions &Options) const;
+
 private:
   friend class CustomAttrNominalRequest;
   void resetTypeInformation(TypeExpr *repr);

lib/AST/ASTPrinter.cpp

@@ -4843,6 +4843,16 @@ void PrintAST::visitMacroExpansionDecl(MacroExpansionDecl *decl) {
   Printer << ')';
 }
 
+void CustomAttr::printCustomAttr(ASTPrinter &Printer, const PrintOptions &Options) const {
+  Printer.callPrintNamePre(PrintNameContext::Attribute);
+  Printer << "@";
+  if (auto type = getType())
+    type.print(Printer, Options);
+  else
+    getTypeRepr()->print(Printer, Options);
+  Printer.printNamePost(PrintNameContext::Attribute);
+}
+
 void PrintAST::visitIntegerLiteralExpr(IntegerLiteralExpr *expr) {
   Printer << expr->getDigitsText();
 }

lib/AST/Attr.cpp

@@ -1453,14 +1453,8 @@ bool DeclAttribute::printImpl(ASTPrinter &Printer, const PrintOptions &Options,
   }
 
   case DeclAttrKind::Custom: {
-    Printer.callPrintNamePre(PrintNameContext::Attribute);
-    Printer << "@";
     auto *attr = cast<CustomAttr>(this);
-    if (auto type = attr->getType())
-      type.print(Printer, Options);
-    else
-      attr->getTypeRepr()->print(Printer, Options);
-    Printer.printNamePost(PrintNameContext::Attribute);
+    attr->printCustomAttr(Printer, Options);
     break;
   }