[flang][hlfir] Add pass to inline elementals

Implement hlfir.elemental inlining as proposed in
flang/docs/HighLevelFIR.md.

This is a separate pass to make the code easier to understand. One
alternative would have been to modify the hlfir.elemental lowering in
the HLFIR bufferization pass.

Currently, a hlfir.elemental can only be inlined once; if there are
more uses, the existing bufferization is used instead.

Usage of mlir::applyPatternsAndFoldGreedily was suggested by @jeanPerier

Differential Revision: https://reviews.llvm.org/D149258