There's still an exception for changing the names of structures, but
otherwise the data stored in the TSymbol base class is never changed.

BUG=angleproject:2267
TEST=angle_unittests

Change-Id: I5b3f44cd365320ad09b46d2cfa3654a7d3f4bbc3
Reviewed-on: https://chromium-review.googlesource.com/806556Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>