I am making a CG pass to depend on `FIROpenACCSupport` in #134346.
This introduces a cyclic dependency between `FIROpenACCSupport`
and `FIRCodeGen`. This patch splits `FIRCodeGen` into
`FIRCodeGenDialect` (for FIR CG dialect definition) and `FIRCodeGen`
(for the CG passes).
Now, `FIROpenACCSupport` depends on `FIRCodeGenDialect`,
and `FIRCodeGen` depends on `FIROpenACCSupport`.