Fix issue #1819 incorrect enum value name generation with leading digits

[TheMinefighter]

Currently any character complying with isJavaIdentifierPart but not isJavaIdentifierStart (e.g. digits) at the beginning of the name of an enum value will make buildCEnumConstants return null (as isJavaIdentifier fails) which causes the enums values to be named VALUE_x (method is called again but with needsToGenerateMemberName).

This PR fixes that by checking whether the first character of an enum value name complies with isJavaIdentifierStart, if not an underscore will be added to the StringBuilder that builds the name of the value. As that check is executed before the normal name generation loop that underscore is the first char of the StringBuilder and acts a prefix to the name. That behavior is consistent with the one found in other places of the java code generation, for example a package called 12 will be renamed to _12.

Note: There was a typo in an earlier version of this PR