Sanitize Record._elements (backport #3419) (#3426)

* Sanitize Record._elements (#3419)

This moves name sanitation a little earlier and ensures that the
sanitized names are use more consistently throughout Chisel internals.

This fixes a bug in D/I where unsanitary names would lead to a crash.

(cherry picked from commit 5c282db5)

* Keep Scala 2.12 support by not using VectorMap

* Fix firrtl in tests for older firrtl spec version

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Co-authored-by: Jack Koenig <koenig@sifive.com>