Shortcuts allow identifiable primary expressions to bypass all productions
starting from assignment expression. We can identify them by looking at the
token after the single token of which the primary expression consists. This CL
adds support for 'identifier' followed by '=' as a primary LHS.

Change-Id: I2f1939a39e03384598359a3a39e1d6cef9967e21
Reviewed-on: https://chromium-review.googlesource.com/1252805Reviewed-by: Camillo Bruni <cbruni@chromium.org>
Commit-Queue: Toon Verwaest <verwaest@chromium.org>
Cr-Commit-Position: refs/heads/master@{#56306}