When mips disassembler process `DecodeMem`, does not consider
Mips::SC64, leading to `Operands.size() = 3`.
And the right `Size` value is 4, when printMemOperand would occur
asserts: `assert(idx < size());`

Fix #157508.

Rearrange decode functions to be before including the generated
disassembler code and eliminate forward declarations for most of them.
This is possible because `fieldFromInstruction` is now in MCDecoder.h
and not in the generated disassembler code.

Extract fixed functions generated by decoder emitter into a new
MCDecoder.h header.

## Purpose

This patch is one in a series of code-mods that annotate LLVM’s public
interface for export. This patch annotates the `llvm/Target` library.
These annotations currently have no meaningful impact on the LLVM build;
however, they are a prerequisite to support an LLVM Windows DLL (shared
library) build.

## Background

This effort is tracked in #109483. Additional context is provided in
[this
discourse](https://discourse.llvm.org/t/psa-annotating-llvm-public-interface/85307),
and 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).

A sub-set of these changes were generated automatically using the
[Interface Definition Scanner (IDS)](https://github.com/compnerd/ids)
tool, followed formatting with `git clang-format`.

The bulk of this change is manual additions of `LLVM_ABI` to
`LLVMInitializeX` functions defined in .cpp files under llvm/lib/Target.
Adding `LLVM_ABI` to the function implementation is required here
because they do not `#include "llvm/Support/TargetSelect.h"`, which
contains the declarations for this functions and was already updated
with `LLVM_ABI` in a previous patch. I considered patching these files
with `#include "llvm/Support/TargetSelect.h"` instead, but since
TargetSelect.h is a large file with a bunch of preprocessor x-macro
stuff in it I was concerned it would unnecessarily impact compile times.

In addition, a number of unit tests under llvm/unittests/Target required
additional dependencies to make them build correctly against the LLVM
DLL on Windows using MSVC.

## Validation

Local builds and tests to validate cross-platform compatibility. This
included llvm, clang, and lldb on the following configurations:

- Windows with MSVC
- Windows with Clang
- Linux with GCC
- Linux with Clang
- Darwin with Clang

Identified with misc-include-cleaner.

Commit a538d1f13a13 first added support for named sub-operands in
CodeEmitterGen. We now add a few more features to that, enabling
further target cleanups.

1. Adds support for handling an EncoderMethod in a sub-operand in
CodeEmitterGen. Previously, the specified encoder of a sub-operand was
ignored, and only the default used.

2. Adds support for sub-operands in DecoderEmitter, along with support
for tied sub-operands.

The changes to the decoder required a few minor tweaks to a few
targets, where existing brokeness was exposed. In order to keep this
patch small, I left FIXMEs which will be addressed in upcoming
patches. (Except MIPS16, since its object file emission/decoding is
totally broken).

Differential Revision: https://reviews.llvm.org/D137653