This CL renames Constant to ExternConstant (this already happend in the
grammar). It also enforces the rule that such extern constants require
"constexpr" types.

Drive-by-change: Replaced non constexpr extern constants with
module constants.

R=tebbi@chromium.org

Bug: v8:7793
Change-Id: Icb3f75071b15b1fcabbe447941e05dd5a09d4b23
Reviewed-on: https://chromium-review.googlesource.com/1136434Reviewed-by: Tobias Tebbi <tebbi@chromium.org>
Commit-Queue: Simon Zünd <szuend@google.com>
Cr-Commit-Position: refs/heads/master@{#54453}