[llvm] revisions to `LLVM_ABI` export macro definitions (#144598)

## Purpose

Simplify the logic used to define `LLVM_ABI` and related macros,
eliminate the `LLVM_ABI_FRIEND` macro, and 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](https://github.com/llvm/llvm-project/blob/main/llvm/docs/InterfaceExportAnnotations.rst).

## Overview
- Define a new CMake config value,
`LLVM_ENABLE_LLVM_EXPORT_ANNOTATIONS`, which is implicitly set whenever
`LLVM_BUILD_LLVM_DYLIB`, `LLVM_BUILD_SHARED_LIBS`, or
`LLVM_ENABLE_PLUGINS` is set. Add it as a `#cmakedefine` to
llvm-config.h so its definition is available to projects building
against LLVM as required so clients see `__declspec(dllimport)` on
Windows.
- Gate the `LLVM_ABI` macro definitions in Compiler.h behind the new
`LLVM_ENABLE_LLVM_EXPORT_ANNOTATIONS` definition. This is
simpler/cleaner, but should be equivalent to the previous logic.
- Maintain `LLVM_BUILD_STATIC` as an override to be used by specific
targets that don't want to build against the DLL/shared library, such as
tablegen.
- For ELF and Mach-O targets, directly define `LLVM_ABI` as
`__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 as static lib with clang and gcc on Linux.
- Built as static with clang-cl and MSVC on Windows.
- Built as shared lib with clang and gcc on Linux (+ additional local
changes not yet merged).
- Built as DLL with clang-cl and MSVC on Windows (+ additional local
changes not yet merged).

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Co-authored-by: SquallATF <squallatf@gmail.com>

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