This CL fixes a crash when "constexpr" is infered from the intializer
expression of a variable declaration.

R=sigurds@chromium.org

Bug: v8:7793
Change-Id: I0ec51280fa145d874424e885905bbf79c93b3904
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1627983
Commit-Queue: Simon Zünd <szuend@chromium.org>
Auto-Submit: Simon Zünd <szuend@chromium.org>
Reviewed-by: Sigurd Schneider <sigurds@chromium.org>
Cr-Commit-Position: refs/heads/master@{#61826}