Use only ImmutableString in TextureFunctionHLSL BUG=angleproject:2267 TEST=angle_unittests, angle_end2end_tests Change-Id: I344ca0098762fcf665365c79d1f8fb04cb1b03f6 Reviewed-on: https://chromium-review.googlesource.com/887067 Reviewed-by: Jamie Madill <jmadill@chromium.org> Reviewed-by: Corentin Wallez <cwallez@chromium.org> Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>