[llvm] [llvm] DLLExport format_object_base (PR #127929)

[Andrew Rogers via llvm-commits]

https://github.com/andrurogerz created https://github.com/llvm/llvm-project/pull/127929

## Overview
Annotate the `llvm::format_object_base` class so it is explicitly included in LLVM's public interface. When building LLVM as a Windows DLL, this annotation exports `format_object_base`'s public methods from the DLL.

## Background
This change is required as part of the overall project to build LLVM as a Windows DLL described in #109483. Without this change, LLVM tools fail to link.

## Validation
Built LLVM with MSVC on Windows 11:
```
cmake -B build -S llvm -G Ninja -DCMAKE_BUILD_TYPE=Debug -DLLVM_ENABLE_PROJECTS="clang" -DLLVM_OPTIMIZED_TABLEGEN -DLLVM_BUILD_LLVM_DYLIB=ON -DLLVM_BUILD_LLVM_DYLIB_VIS=ON -DLLVM_LINK_LLVM_DYLIB=ON -DCLANG_LINK_CLANG_DYLIB=ON
ninja -C build llvm-rc llvm-ar
```
Verified the annotated interface no longer appears in the list of unresolved external symbols.

>From b925f7a027bc7bed683f73e44d7bdc3e6f0a1a7f Mon Sep 17 00:00:00 2001
From: Andrew Rogers <andrurogerz at gmail.com>
Date: Wed, 19 Feb 2025 16:23:13 -0800
Subject: [PATCH] DLLExport format_object_base

---
 llvm/include/llvm/Support/Format.h | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/llvm/include/llvm/Support/Format.h b/llvm/include/llvm/Support/Format.h
index 89b6ae35ba5de..2553002b37899 100644
--- a/llvm/include/llvm/Support/Format.h
+++ b/llvm/include/llvm/Support/Format.h
@@ -25,6 +25,7 @@
 #include "llvm/ADT/ArrayRef.h"
 #include "llvm/ADT/STLExtras.h"
 #include "llvm/ADT/StringRef.h"
+#include "llvm/Support/Compiler.h"
 #include "llvm/Support/DataTypes.h"
 #include <cassert>
 #include <cstdio>
@@ -36,7 +37,7 @@ namespace llvm {
 
 /// This is a helper class used for handling formatted output.  It is the
 /// abstract base class of a templated derived class.
-class format_object_base {
+class LLVM_ABI format_object_base {
 protected:
   const char *Fmt;
   ~format_object_base() = default; // Disallow polymorphic deletion.