This removes ibm_db, informix, mssql, oracle, and oracle12 Arel visitors
which are not used in the code base.

Actually oracle and oracle12 visitors are used at oracle-enhanced
adapter, but now I think that those visitors should be in the adapter's
repo like sqlserver adapter and the dedicated Arel visitor
(https://github.com/rails-sqlserver/activerecord-sqlserver-adapter/blob/master/lib/arel/visitors/sqlserver.rb),
otherwise it is hard to find a bug and review PRs for the oracle
visitors (e.g. #35838, #37646), since we don't have knowledge and
environment enough for Oracle.

use number instead of string in application.rb in Rails guide

Raise template errors on correct line with annotations enabled

Co-authored-by: NAaron Patterson <tenderlove@github.com>

Correct ruby-pg repo link in rdocs

Avoid ActionView rendering instrumentation indirection

Improve ActiveSupport Parallelization error logging

Prevent has_one from touching parent record unless persisted

Update SchemaCacheTest#schema_dump_path

Remove dead test code for unsupported adapters

  * Use ASSETS_ROOT (defined in activerecord/test/config.rb) to guarantee a
    valid path to schema_dump_5_1.yml.

Previously, if `build_association` was called multiple times for a `has_one` association but never committed to the database, the first newly-associated record would trigger `touch` during the attempted removal of the record.

For example:

    class Post < ActiveRecord::Base
      has_one :comment, inverse_of: :post, dependent: :destroy
    end

    class Comment < ActiveRecord::Base
      belongs_to :post, inverse_of: :comment, touch: true
    end

    post = Post.create!
    comment_1 = post.build_comment
    comment_2 = post.build_comment

When `comment_2` is initialized, the `has_one` would attempt to destroy `comment_1`, triggering a `touch` on `post` from an association record that hasn't been committed to the database.

This removes the attempt to delete an associated `has_one` unless it’s persisted.

This deprecation is useless since the result is still an Error object
and there is no way to fix the code to remove the deprecation.

Let's just accept as a breaking change.

Followup to e7514dc6.

Ignore Errno::ENOTEMPTY when calling AS::Cache::FileStore#clear with race conditions

Load XML Builder if it is not available

Document update for ActiveModel#errors

Fix perform_enqueued_jobs without a block inside other helpers

assert_enqueued_with with a block ignores all the jobs enqueued before
the block for its assertions by counting the number of jobs and dropping
the n first elements from the Array, but since we're now mutating the
Array in perform_enqueued_jobs without a block, it's broken.

This uses another implementation which is correct when the array is
mutated, by getting a duplicated array of jobs, then removing them from
the original array.

Similarly assert_enqueued_jobs with a block was using counts only, now
keeps track of the specific jobs to count them at the end.

Reduce calls to get path info when finding routes

Followup to b94efe9f.

Fix random CI fail due to non-deterministic order

Example failure: https://buildkite.com/rails/rails/builds/68187#333e3624-ac0d-4b23-95b9-f068d9901093/1016-1028

These tests expect the 2nd Account fixture (with a NULL `firm_id`) to be
in the first four rows of the result set, but that behavior is not
guaranteed without an `order`.

FrontBase and IBM_DB adapters do not work for Rails 5.2.

https://rubygems.org/gems/ruby-frontbase
https://rubygems.org/gems/ibm_db

Remove reference to global rails command

Related #15137.

Firebird related code is already removed in #15137.

We have two `current_adapter?(:DB2Adapter)` in tests, but the adapter is
no longer maintained (last release is November 15, 2012).

https://rubygems.org/gems/db2

Yet another (latest) DB2 adapter (`IBM_DBAdapter`) might support Rails
5.0.7, but apparently do not work for Rails 5.2.

https://rubygems.org/gems/ibm_db

We have few lines mention about DB2 in the doc, but now there is no
worth for almost all current users.

Do not expose association relation in the API doc [ci skip]

Allow extra scoping in callbacks when create on association relation

Remove dead code in tests

Add subsec to `ActiveSupport::TimeWithZone#inspect`