[flang] Handle array constants of any rank

Add support for representing array constants of any rank with MLIR
dense attribute. This greatly improves compile time and memory
usage of programs with large array constants. We still support only
arrays of a few basic types, such as integer, real and logic.

Fixes https://github.com/llvm/llvm-project/issues/60376

Reviewed By: jeanPerier

Differential Revision: https://reviews.llvm.org/D150686