Farewell Scala 2.11

* Add (failing) Test for Data toTarget calls

Add scaladoc and clean up test

* Builder: don't let .toTarget pass if it won't be able to deserialize properly later

* Update src/test/scala/chiselTests/ReferenceTargetSpec.scala

* Rename and simplify tests for literal toTarget

Co-authored-by: mergify[bot] <37929162+mergify[bot]@users.noreply.github.com>

MiMa binary compatibility checks are now run on master, but the
mimaPreviousArtifacts are empty which makes the check a no-op. This
helps keep both the build.sbt and CI more consistent between master and
stable branches.

.mergify.yml is also updated with mergify.sc from chisel-repo-tools
using the following configuration:

conditions:
  - status-success=all tests passed
branches:
  - 3.2.x
  - 3.3.x
  - 3.4.x

- fix build.sbt
- fix build.sc

* Builder: use LazyLogging.logger.warn to print elaboration message

* add deprecation for chisel3.internal.ErrorLog.info.

* add test to check elaboration message still exist.

Co-authored-by: Kevin Laeufer <kevin.laeufer@sifive.com>

Implemented by folding Element.ref into Data.ref. Element.ref had
special handling for literals, but because Bundles can also be literals,
there were code paths that tried to get the ref of a Bundle literal
which was non-existent. Now, all literals are handled together.

Because FIRRTL does not have support for Bundle literals, Bundle literal
refs are implemented by materializing a Wire.

* Improve source locators for switch statements.

The SecurityManager is global so is not thread-safe. This is the source
of flaky tests in FIRRTL CI.

Co-authored-by: mergify[bot] <37929162+mergify[bot]@users.noreply.github.com>

* Refine autonaming to have more intuitive behavior

Last name in an Expression wins, while the first Statement to name wins.
This is done via checking the _id of HasIds during autonaming and only
applying a name if the HasId was created in the scope of autonaming.
There is no change to .autoSeed or .suggestName behavior.

Behavior of chisel3-plugins from before this change is maintained.

* Update docs with naming plugin changes

This is a refinement of the assertion added in #1616 then removed in
 #1654. Because Records now set the refs of children upon binding,
later, unbound Records could incorrectly override the refs. The first
set should win.

It causes issues for some legal (if awkward) patterns. A larger refactor
of when refs are set could reinstate this check.

Co-authored-by: mergify[bot] <37929162+mergify[bot]@users.noreply.github.com>

This requires a combination of things, but it happens to be a
combination used by Diplomacy in Rocket Chip. It must be a Record in
compatibility code with Vecs as fields and a mix of components with and
without set directions.