This fixes an issue in the original revision of this patch by adding
a operator<< to TInfoSinkBase that takes ImmutableString as a
parameter.

This also adds ImmutableStringBuilder class, which can be used to
build ImmutableStrings in place without extra allocations if the
maximum length is known in advance.

BUG=angleproject:2267
TEST=angle_unittests

Change-Id: I52e984657a3aba3e6fe67a82b401c6b8de557d18
Reviewed-on: https://chromium-review.googlesource.com/890522Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>