[llvm] updates to LLVM_ABI export macro definitions

[andrurogerz]

Purpose

Update the LLVM_ABI macro to always resolve to __attribute__((visibility("default"))) when building LLVM as a shared library for ELF or Mach-O targets.

Background

Previously, LLVM_ABI was defined to the C++ style attribute [[gnu::visibility("default")]] when compiling with gcc, which has more restrictions on its placement. Of note, the C++ style attributes cannot decorate friend functions and must not appear after extern on variable declarations.

Documentation for LLVM_ABI and related annotations is found in the LLVM repo here.

Overview

• Directly define LLVM_ABI to __attribute__((visibility("default"))) instead of LLVM_ATTRIBUTE_VISIBILITY_DEFAULT, which resolves to C++ style [[gnu::visibility("default")]] when compiling with gcc.
• Remove the LLVM_ABI_FRIEND macro and replace all usages of it with LLVM_ABI.
• Update the documentation for exporting friend functions to no longer reference LLVM_ABI_FRIEND.

Validation

Built with clang and gcc on Linux.