This ensures that pointers to Macros that are removed from the macro
set stay valid. Pointers to undef'd macros may need to be referred to
if reenabling the macros has been deferred.

BUG=chromium:681324
TEST=angle_unittests

Change-Id: Ibbbabbcbd6b0a84254cda717ae63712e6d404ebd
Reviewed-on: https://chromium-review.googlesource.com/427948Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>