[flang] Move Details rather than copying

We don't need to copy the various Details classes. We will be adding
expressions to some of them (e.g. for bounds or initial values) and they
should generally be moved as well.

In check-do-concurrent.cc, put pointers to Symbols in the symbol
collections rather than copies.

Original-commit: flang-compiler/f18@cdedfc9b3ee554bf6b1f94adbbb92f8a515e1cbd
Reviewed-on: https://github.com/flang-compiler/f18/pull/223
Tree-same-pre-rewrite: false