[SandboxVec] SandboxVectorizerIR Boilerplate (#189515)
This patch introduces a new specialization of SandboxIR named
SandboxVectorizerIR that contains the new Pack instruction (though it is
just a placeholder for now). It also implements the necessary
boilerplate in SandboxIR to support adding such specializations.
Each specialization requires its own *Values.def file defining its new
instructions. The new .def file also needs to be added to
ValuesDefFilesList.def which allows SandboxIR to include entries from
all specializations and populate structures like the Opcode enum,
ClassID and others.
Given that we need to include both .def files in several places, I had
to (i) change the .def file to undefine its locally defined macros so
that they won't get redefined in the next #include .def, and (ii) add
the DEF_DISABLE_AUTO_UNDEF option to disable undefining the macros at
the end because otherwise the user-specified DEF_ macros would be
undefined by the time we reach the second .def file.
