[mlir][spirv] Enable serializer to write SPIR-V modules into separate files (#152678)

By default, `mlir-translate` writes all output into a single file even
when `--split-input-file` is used. This is not an issue for text files
as they can be easily split with an output separator. However, this
causes issues with binary SPIR-V modules.

Firstly, a binary file with multiple modules is not a valid SPIR-V, but
will be created if multiple modules are specified in the same file and
separated by "// -----". This does not cause issues with MLIR internal
tools but does not work with SPIRV-Tools.

Secondly, splitting binary files after serialization is non-trivial,
when compared to text files, so using an external tool is not desirable.

This patch adds a SPIR-V serialization option that write SPIR-V modules
to separate files in addition to writing them to the `mlir-translate`
output file. This is not the ideal solution and ideally `mlir-translate`
would allow generating multiple output files when `--split-input-file`
is used, however adding such functionality is again non-trival due to
how processing and splitting is done: output is written to a
single `os` that is passed around, and the number of split buffers is not
known ahead of time. As such a I propose to have a SPIR-V internal
option that will dump modules to files in the form they can be processed
by `spirv-val`. The behaviour of the new added argument may be
confusing, but benefits from being internal to SPIR-V target.

Alternatively, we could expose the spirv option in
`mlir/lib/Tools/mlir-translate/MlirTranslateMain.cpp`, and slice the
output file on the SPIR-V magic number, and not keep the file generated
by default by `mlir-translate`. This would be a bit cleaner in API
sense, as it would not generate the additional file containing all
modules together. However, it pushes SPIR-V specific code into the
generic part of the `mlir-translate` and slicing is potentially more
error prone that just writing a single module after it was serialized.