Update DWARF for space optimizations

[tromey]

#45225 implemented a number of space optimizations. However, the DWARF was not updated to follow.

For example, for a sample type mentioned in the PR:

enum E { A(bool), B, C, D }

Nightly emits this DWARF:

 <2><b9>: Abbrev Number: 6 (DW_TAG_union_type)
    <ba>   DW_AT_name        : (indirect string, offset: 0x8a275): E
    <be>   DW_AT_byte_size   : 1
    <bf>   DW_AT_alignment   : 1
 <3><c0>: Abbrev Number: 7 (DW_TAG_member)
    <c1>   DW_AT_name        : (indirect string, offset: 0x7d): RUST$ENCODED$ENUM$0$B
    <c5>   DW_AT_type        : <0xcc>
    <c9>   DW_AT_alignment   : 1
    <ca>   DW_AT_data_member_location: 0
 <3><cb>: Abbrev Number: 0
 <2><cc>: Abbrev Number: 8 (DW_TAG_structure_type)
    <cd>   DW_AT_name        : (indirect string, offset: 0x7092a): A
    <d1>   DW_AT_byte_size   : 1
    <d2>   DW_AT_alignment   : 1
 <3><d3>: Abbrev Number: 7 (DW_TAG_member)
    <d4>   DW_AT_name        : (indirect string, offset: 0x93): __0
    <d8>   DW_AT_type        : <0xe0>
    <dc>   DW_AT_alignment   : 1
    <dd>   DW_AT_data_member_location: 0

This is entirely missing entries for C and D (B is implicit in the special RUST$ENCODED$ENUM$0$B member name).

This is a good opportunity to remove RUST$ENCODED$ENUM and also RUST$ENUM$DISR.

@eddyb pointed out one approach on irc, namely to use the existing DW_TAG_variant_part support in DWARF. This will yield somewhat weird-looking output; first because the tag is only defined as a child of a structure, so the union will have to be wrapped in an artificial struct; and second because the discriminant is defined to be a reference to one of the struct's members.

This does seem likely to work, though; and since neither gdb nor lldb currently handle DW_TAG_variant_part at all, we probably have some leeway to do what we like. Plan B might be something like introduce a small DWARF extension to represent the layout in a form closer to what rustc is actually doing.

Unfortunately LLVM's debuginfo builder also does not seem to have a notion of discriminated unions, so step 1 will have to be an LLVM patch.

[Gankra]

Linking from IRC discussion. Swift has its own fork/copy of lldb, and includes aggressively (dynamically!) optimized enums: https://github.com/apple/swift-lldb

[tromey]

I think this bug is a dup of #32920

[tromey]

I belatedly realized I could close this. Dup of #32920