[openacc] Add acc routine support to acc dialect

Adds representation for `acc routine` under new operation named
`acc.routine`. This operation is associated with a function symbol.
It also gets its own compiler generated synthetic symbol name so
that it can be referenced from the associated function. The clauses
associated with the `acc routine` directive are captured in the
`acc.routine` op.

The linking between the `func.func` and its `acc.routine` declaration
is done through the `acc.routine_info` attribute. In practice, a
single `acc routine` is associated with a function. But the spec does
not specifically restrict this - thus the 1:N relationship between
`func.func` and `acc.routine` allowed in the dialect. Additionally, it
makes sense that multiple acc routines could be used for a single
function depending on loop context - to allow flexible parallelization.

Most acc routine clauses are supported including `gang`, `gang(dim:)`,
`vector`, `worker`, `seq`, `nohost`, and `bind`. The only one not
supported is `device_type`. This is because most other clauses also
miss this and the effort to add support for it needs to be coordinated
and consistent.

Reviewed By: clementval, vzakhari

Differential Revision: https://reviews.llvm.org/D156281