[flang][fir] Add array value operations.

We lower expressions with rank > 0 to a set of high-level array operations.
These operations are then analyzed and refined to more primitve
operations in subsequent pass(es).

This patch upstreams these array operations and some other helper ops.

Authors: Eric Schweitz, Rajan Walia, Kiran Chandramohan, et.al.

https://github.com/flang-compiler/f18-llvm-project/pull/565

Differential Revision: https://reviews.llvm.org/D97421