[llvm] [llvm] ensure DEMANGLE_ABI is properly defined (PR #151023)

[Andrew Rogers via llvm-commits]

https://github.com/andrurogerz created https://github.com/llvm/llvm-project/pull/151023

## Overview
Include  `llvm-config.h` from `DemangleConfig.h` so `LLVM_ENABLE_LLVM_EXPORT_ANNOTATIONS` is defined correctly. The presence of this definition controls the definition of `LLVM_ABI` on Windows DLL builds. This include was missed in #147564.

## Background
This effort is tracked in #109483. Additional context is provided in [this discourse](https://discourse.llvm.org/t/psa-annotating-llvm-public-interface/85307).

>From 5252eda5ad1a11ab9481738e204b7e75555e14bf Mon Sep 17 00:00:00 2001
From: Andrew Rogers <andrurogerz at gmail.com>
Date: Mon, 28 Jul 2025 11:36:05 -0700
Subject: [PATCH] ensure DEMANGLE_ABI is properly defined

---
 llvm/include/llvm/Demangle/DemangleConfig.h | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/llvm/include/llvm/Demangle/DemangleConfig.h b/llvm/include/llvm/Demangle/DemangleConfig.h
index 7ee23a4546377..912c9b869bf7d 100644
--- a/llvm/include/llvm/Demangle/DemangleConfig.h
+++ b/llvm/include/llvm/Demangle/DemangleConfig.h
@@ -15,6 +15,9 @@
 #ifndef LLVM_DEMANGLE_DEMANGLECONFIG_H
 #define LLVM_DEMANGLE_DEMANGLECONFIG_H
 
+// llvm-config.h is required for LLVM_ENABLE_LLVM_EXPORT_ANNOTATIONS
+#include "llvm/Config/llvm-config.h"
+
 #ifndef __has_feature
 #define __has_feature(x) 0
 #endif