[llvm] add TODO and link to GitHub issue per PR feedback

Author: andrurogerz

llvm/include/llvm/Support/Compiler.h

@@ -179,6 +179,8 @@
 // Marker to add to classes or functions in public headers that should not have
 // export macros added to them by the clang tool
 #define LLVM_ABI_NOT_EXPORTED
+// TODO(https://github.com/llvm/llvm-project/issues/145406): eliminate need for
+// two preprocessor definitions to gate LLVM_ABI macro definitions.
 #if defined(LLVM_ENABLE_LLVM_EXPORT_ANNOTATIONS) && !defined(LLVM_BUILD_STATIC)
 #if defined(_WIN32) && !defined(__MINGW32__)
 #if defined(LLVM_EXPORTS)