PERF: optimise type lookup to avoid invoking procs

Mostly resolves #17641

```
require 'benchmark/ips'
require 'stackprof'

require File.expand_path("../../config/environment", __FILE__)

connection = ActiveRecord::Base.connection

sql = "select #{(1..10).map{|i| "#{i} #{(96+i).chr}" }.join(", ")}"
puts "SQL is #{sql}"

Benchmark.ips do |b|
  b.report("select_all") do |times|
    while times > 0
      connection.select_all(sql)
      times -= 1
    end
  end
end
```


cc @sgrif @tenderlove 

Before:

```
sam@ubuntu discourse % RAILS_MASTER=1 RAILS_ENV=production ruby script/select_all_bench.rb
SQL is select 1 a, 2 b, 3 c, 4 d, 5 e, 6 f, 7 g, 8 h, 9 i, 10 j
Calculating -------------------------------------
          select_all   328.000  i/100ms
-------------------------------------------------
          select_all      3.337k (± 5.5%) i/s -     16.728k

```

after 

```
SQL is select 1 a, 2 b, 3 c, 4 d, 5 e, 6 f, 7 g, 8 h, 9 i, 10 j
Calculating -------------------------------------
          select_all   507.000  i/100ms
-------------------------------------------------
          select_all      5.409k (± 5.1%) i/s -     27.378k
```

So we are still a touch slower than 4.1 but almost there
Why not put this in the body of the fetch block?