Merge the documentation of the definition of extensible dialects
with the definition of dialects.
Reviewed By: rriddle
Differential Revision: https://reviews.llvm.org/D125200
We don't actually have any documentation today for how to
declaratively define a dialect. This commit rectifies that and properly
documents how to define a Dialect in tablegen, and details all of
the possible fields.
Differential Revision: https://reviews.llvm.org/D123258