preprocessor: Fix use after free when #undef the macro being invoked BUG=chromium:648031 BUG=angleproject:1522 Change-Id: I825cea9e736a2c99133408249cfcd525431d31de Reviewed-on: https://chromium-review.googlesource.com/386853 Commit-Queue: Corentin Wallez <cwallez@chromium.org> Reviewed-by: Jamie Madill <jmadill@chromium.org> Reviewed-by: Geoff Lang <geofflang@chromium.org>