This CL adds a 'constexpr String' type to Torque, which gets compiled
to 'const char*'.

It makes it easier to pass strings around in Torque files,
especially when trying to Print() them.

Bug: v8:7793
Change-Id: I2d4feac7012f5401f2b467475f8df086e46ac18a
Reviewed-on: https://chromium-review.googlesource.com/1104466
Commit-Queue: Théotime Grohens <theotime@google.com>
Reviewed-by: Tobias Tebbi <tebbi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#53814}