tblgen be non-const. This requires introducing some const_cast's at the
moment, but those (and lots more stuff) will disappear in subsequent patches.
This significantly simplifies those patches because the various tblgen op emitters
get adjusted.
PiperOrigin-RevId: 239954566
Previously we emit both op declaration and definition into one file and include it
in *Ops.h. That pulls in lots of implementation details in the header file and we
cannot hide symbols local to implementation. This CL splits them to provide a cleaner
interface.
The way how we define custom builders in TableGen is changed accordingly because now
we need to distinguish signatures and implementation logic. Some custom builders with
complicated logic now can be moved to be implemented in .cpp entirely.
PiperOrigin-RevId: 239509594
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