[analyzer] LSP 'get implementations': get lineInfo directly

Asking for implementations of 'ToJsonable' from
package:analysis_server/lsp_protocol/protocol_special.dart takes
8+ seconds both on first request and on subsequent requests.

Turns out most of that is spent on getting line info data for each
result via `server.getLineInfo`.

This CL rewrites how the search is performed, getting all subtypes
directly via `searchAllSubtypes`, from which one can cheaply get the
line info.

On my machine, the call that always took 8+ seconds before now takes
<2 seconds (at least on subsequent runs).
Notice that the time also depends on how much is open by the editor and
that calls from my editor with many folders open still takes ~6 seconds.

This CL furthermore adds more performance measurements that's available
via the "Analysis Server Diagnostics" page "Timing" for
"textDocument/implementation" requests.

Change-Id: I2e5ef20238901fb4734cc182aeb53adfc96f79b6
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/272360
Commit-Queue: Jens Johansen <jensj@google.com>
Reviewed-by: Konstantin Shcheglov <scheglov@google.com>