In the current implementation, compilation would fail because
operator<< is not defined for enum classes. For others, the compiler
finds more than one operator<<, so it fails because it's ambiguous.

This CL fixes this by printing the integer value for enums, uses the
operator<< for all values that support it, and prints "<unprintable>"
otherwise.

Also, lots of unit tests.

R=ishell@chromium.org

Bug: v8:6837
Change-Id: I895ed226672aa07213f9605e094b87af186ec2e4
Reviewed-on: https://chromium-review.googlesource.com/671016
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Igor Sheludko <ishell@chromium.org>
Cr-Commit-Position: refs/heads/master@{#48110}