This is a reland of 0a59e0cb
which was a reland of 146f5375
which was a reland of d91679bf

Initializes the BackgroundCompileTasks's language_mode in the
constructor (previously only initialized after successful parse) in case
the parse failed. We still need to reset it after parse in case the
language mode changed (because we encountered "use strict").

Original change's description:
> [parser] Introduce UnoptimizedCompileFlags
>
> UnoptimizedCompileFlags defines the input flags shared between parse and
> compile (currently parse-only). It is set initially with some values, and
> is immutable after being passed to ParseInfo (ParseInfo still has getters
> for the fields, but no setters).
>
> Since a few of the existing flags were output flags, ParseInfo now has a
> new output_flags field, which will eventually migrate to a ParseOutputs
> structure.
>
> Bug: v8:10314
> Change-Id: If3890a5fad883bca80a97bf9dfe44d91797dc286
> Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2096580
> Commit-Queue: Leszek Swirski <leszeks@chromium.org>
> Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
> Reviewed-by: Simon Zünd <szuend@chromium.org>
> Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#66782}

TBR=ulan@chromium.org,szuend@chromium.org,rmcilroy@chromium.org

Bug: v8:10314
Change-Id: Ieee0bbfade4fe0b56de03bff47a7364959608d6a
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2157367Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Commit-Queue: Leszek Swirski <leszeks@chromium.org>
Cr-Commit-Position: refs/heads/master@{#67265}