Avoid duplicate warnings about %error-verbose, once for deprecation,
another for duplicate. Keep only the duplicate warning for the second
occurrence of %error-verbose.
This will help removal fixits.
* src/scan-gram.l (%error-verbose): Return as a PERCENT_ERROR_VERBOSE
token.
* src/parse-gram.y (do_error_verbose): New.
Use it.
* src/muscle-tab.c (muscle_percent_variable_update): Handle pseudo
variables such as %error-verbose.
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