Fix #20004 - generate DW_TAG_enumerator for enum members in DWARF deb…

[dkorpel]

…ug info

Closes #20004

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[dkorpel]

The backend currently assumes TYenum is a 4-byte integer (like in C), it doesn't support D's arbitrary enums. Adding TYenum8 TYenum16 types etc. looks redundant, maybe the backend type of D enums should always be whatever the base type is (TYuns8 TYuns16 etc.) with a separate flag specifying it has an enum symbol for debug info.

[ibuclaw]

@dkorpel dwarf supports this (enum class)

[ibuclaw]

From the dwarfv5 spec (rewrote example as D)

enum E : byte { E1 , E2 = 100 }

! DWARF description !
11$: DW_TAG_enumeration_type
        DW_AT_name("E")
        DW_AT_type(reference to "byte")
        DW_AT_enum_class(present)
12$:    DW_TAG_enumerator
            DW_AT_name("E1")
            DW_AT_const_value(0)
13$:    DW_TAG_enumerator
            DW_AT_name("E2")
            DW_AT_const_value(100)
14$: 

[ibuclaw]

Named enums in D are all strongly typed, so DW_AT_enum_class should always be emitted in debug code.

[dkorpel]

The dwarf debug info generation works with the current changes, but the change from TYuns8 to TYenum causes problems with code generation