[flang][hlfir] Codegen for hlfir.get_length.

Lower hlfir.get_length into the char length inquiry of the bufferized
entity. In some cases the codegen will fail with
`hlfir.associate of hlfir.expr with more than one use` - this will be fixed
separately (after D154521).

Depends on D154560

Reviewed By: tblah, jeanPerier

Differential Revision: https://reviews.llvm.org/D154561