[llvm] ensure DEMANGLE_ABI is properly defined (#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).

Author: andrurogerz

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