[llvm] DLLExport identify_magic

[andrurogerz]

Overview

Annotate both overloads of the llvm::identify_magic function so they are explicitly included in LLVM's public interface. When building LLVM as a Windows DLL, this annotation exports them 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.

[nikic]

Do not submit these one by one. Consolidating on the discussion in #127850.