False-positive diagnostics due to AstParser::validateLutIndex()

validateLutIndex is being used in AstParser::parseExtendedTable to detect language violations. However, parseExtendedTable requires at least one enumeration value to match for the input to be correct, while validateLutIndex reports "errors" eagerly. The behaviour results in false-positive diagnostics for LUTs with multiple variables. To reproduce, run libertyc on the file attached[1], the sample log included[2].

1. sky130_ef_io__gpiov2_pad_tt_tt_025C_1v80_3v30.lib

2.test.log