Change the coverage measurement logic in the lemon-generated parser so that

Change the coverage measurement logic in the lemon-generated parser so that
it only checks for coverage of state/lookahead pairs that are valid syntax.
It turns out that some states are unreachable if the lookahead is not valid
syntax, because the states are only reachable through a shift following a
reduce, and the reduce does not happen if the lookahead is a syntax error.

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