Fix bugs in 'val and 'pos with range types

Fix bugs in 'val and 'pos with range types

In Ada, the 'val and 'pos attributes can be used to map from an
enumeration constant to its position in the enum and vice versa.
These operators did not work properly when the type in question was a
subrange of an enum type with "holes".

gdb/ChangeLog
2020-05-26  Tom Tromey  <tromey@adacore.com>

	* ada-lang.c (value_val_atr): Handle TYPE_CODE_RANGE.
	* gdbtypes.c (discrete_position): Handle TYPE_CODE_RANGE.

gdb/testsuite/ChangeLog
2020-05-26  Tom Tromey  <tromey@adacore.com>

	* gdb.ada/arr_acc_idx_w_gap.exp: Add enum subrange tests.
	* gdb.ada/arr_acc_idx_w_gap/enum_with_gap.ads (Enum_Subrange): New
	type.
	* gdb.ada/arr_acc_idx_w_gap/enum_with_gap_main.adb (V): New
	variable.