It was switched from a function pointer to std::function in

TableGen: Make 2nd arg MainFn of TableGenMain(argv0, MainFn) optional.
f675ec6165ab6add5e57cd43a2e9fa1a9bc21d81

but there's no mention of any particular reason for that.

Fixes https://github.com/llvm/llvm-project/pull/167700 to support
builds where TableGen's output file is specified as full path
rather than just filename.

Reduces memory usage compiling backend sources, most notably for
AMDGPU by ~98 MB per source on average.

AMDGPUGenRegisterInfo.inc is tens of megabytes in size now, and
is even larger downstream. At the same time, it is included in
nearly all backend sources, typically just for a small portion of
its content, resulting in compilation being unnecessarily
memory-hungry, which in turn stresses buildbots and wastes their
resources.

Splitting .inc files also helps avoiding extra ccache misses
where changes in .td files don't cause changes in all parts of
what previously was a single .inc file.

It is thought that rather than building on top of the current
single-output-file design of TableGen, e.g., using `split-file`,
it would be more preferable to recognise the need for multi-file
outputs and give it a proper first-class support directly in
TableGen.

Most `SourceMgr` clients don't make use of include files, but those that
do might want to specify the file system to use. This patch enables that
by making it possible to pass a `vfs::FileSystem` instance into
`SourceMgr`.

- Move standalone functions and variables out of anonymous namespace and
make them static.
- Eliminate `namespace llvm {}` wrapping all code in .cpp files, and
instead use namespace qualifier to define such functions
(https://llvm.org/docs/CodingStandards.html#use-namespace-qualifiers-to-implement-previously-declared-functions)
- Add namespace for X86DisassemblerShared.h.

This mainly transitions the LLVM intrinsic string table from character
emission to string literal emission, which I confirmed happens for me
locally.

I moved the guts of StringToOffsetTable to a cpp file so I could move
the `EmitLongStrLiterals` cl::opt global to a non-vague linkage home in
the `TableGen` library. I had to add missing FormatVariadic.h includes
to account for moving other includes to a cpp file.

- Bail out of TableGen if any asserts fail before running the backend. 
- Add asserts to validate that the `Objects` and `Modes` lists for
various `HwModeSelect` subclasses are of same length.
 - Eliminate equivalent check in CodeGenHWModes.cpp

Factor out the timer related functionality from `RecordKeeper` to a new
`TGTimer` class in a new file.

- Refactor TableGen backend options to allow specifying a callback
  function that takes either a const reference or a non-const reference
  to `RecordKeeper`. This will enable gradual migration of code to use
  const references and pointers to `RecordKeeper` and  `Record` in the
  TableGen backends. 

- Refactor handling of the callback command line options. Move variable
  for the command line option from the header to the CPP file, and add a
  function `ApplyCallback` to apply the selected callback.

- Change callbacks in TableGen.cpp to take const reference. They use the
  `Opt` class to register their callbacks.

- Change IntrinsicEmitter to use the `OptClass` to define its callbacks.
  It already uses a const reference in the implementation.