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.

Since `raw_string_ostream` doesn't own the string buffer, it is
desirable (in terms of memory safety) for users to directly reference
the string buffer rather than use `raw_string_ostream::str()`.

Work towards TODO comment to remove `raw_string_ostream::str()`.

Identified with clangd.

`Opt(flag, func, desc)` registers an option into `Action`.

`OptClass<EmitterC>` is also available if `EmitterC(RK).run(OS)` is capable.

`Action` is defined as `ManagedStatic<cl::opt>` to guarantee to be created
when each registration of emitter is invoked.

`llvm::TableGenMain(argv0, MainFn)` invokes `Action` instead of `MainFn`

Differential Revision: https://reviews.llvm.org/D144351

Based on the output of include-what-you-use. No other library seems affected by
the new forward declaration.

$ clang++ -E  -Iinclude -I../llvm/include ../llvm/lib/TableGen/*.cpp -std=c++14 -fno-rtti -fno-exceptions | wc -l
before: 795231
after: 750654

Related Discourse thread: https://llvm.discourse.group/t/include-what-you-use-include-cleanup
Differential Revision: https://reviews.llvm.org/D118374