Use the PendingCompilationErrorHandler in the UnoptimizedCompileState
class to prepare compilation errors off-thread, and report them during
the merge into the main thread.

Bug: chromium:1011762
Change-Id: I3ad5078e25c176aa30743500714b2fad838d3ce8
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2105354
Commit-Queue: Leszek Swirski <leszeks@chromium.org>
Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#67353}