This changeset suppresses backtrace locations like
`<internal:array>:211` as much as possible.

Before the patch:
```
$ ruby -e '[1].fetch_values(42)'
<internal:array>:211:in 'Array#fetch': index 42 outside of array bounds: -1...1 (IndexError)
        from <internal:array>:211:in 'block in Array#fetch_values'
        from <internal:array>:211:in 'Array#map!'
        from <internal:array>:211:in 'Array#fetch_values'
        from -e:1:in '<main>'
```

After the patch:
```
$ ./miniruby -e '[1].fetch_values(42)'
-e:1:in 'Array#fetch_values': index 42 outside of array bounds: -1...1 (IndexError)
        from -e:1:in '<main>'
```

Specifically:

* The special backtrace handling of BUILTIN_ATTR_C_TRACE is now always
  applied to frames with `<internal:...>`.
* When multiple consecutive internal frames appear, all but the bottom
  (caller-side) frame are removed.

[Misc #20968]