[sv-bc] constant_select and range BNF fixes

Attached are BNF fixes for a bug in constant_select,
a redundancy in clocking_expression, and missing
multidimensionality in port_reference.

Additionally, there is some cleanup, including
removal of the following nonterminals --

  msb_constant_expression
  lsb_constant_expression
  constant_base_expression
  base_expression
  width_constant_expression
  dimension_constant_expression

-- Brad