Clean up isolate inference and non-allocating/allocating parts of
SharedFunctionInfo::InitFromFunctionLiteral, so that it can more easily
be refactored for off-thread allocation in the future. Also, make
SharedFunctionInfo::SetScript a member function, to acknowledge that it
is non-allocating.

Bug: chromium:1011762
Change-Id: I26624fceb642dfdf257ec3d96aab31ea90e48870
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1991482
Commit-Queue: Leszek Swirski <leszeks@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Auto-Submit: Leszek Swirski <leszeks@chromium.org>
Cr-Commit-Position: refs/heads/master@{#65639}