[flang] Refactor to remove move semantics for Details

The classes in the Details union don't own resources so it makes things
simpler to copy them instead of moving. Same with ProcInterface.

Original-commit: flang-compiler/f18@d9ee19cbc4a8506ab1c34af22a096314782a3354
Reviewed-on: https://github.com/flang-compiler/f18/pull/160
Tree-same-pre-rewrite: false