DWARF puts enum and union with same name into the same scope

[tromey]

This is perhaps a bit of a gdb-specific bug. The basic issue is that having multiple entities in the same scope with the same name but different DWARF tags makes it a pain for gdb to sensibly create its symbol table.

I see the issue with a number of constructs. For this example, consider something like:

enum MoreComplicated {
    One,
    Two(i32),
}

The DWARF gets an enum:

 <2><6d>: Abbrev Number: 4 (DW_TAG_enumeration_type)
    <6e>   DW_AT_type        : <0x415>
    <72>   DW_AT_enum_class  : 1
    <72>   DW_AT_name        : (indirect string, offset: 0x16e): MoreComplicated
    <76>   DW_AT_byte_size   : 1

... but also a union:

 <2><a7>: Abbrev Number: 6 (DW_TAG_union_type)
    <a8>   DW_AT_name        : (indirect string, offset: 0x16e): MoreComplicated
    <ac>   DW_AT_byte_size   : 24

It would be better for gdb to unify these two objects into a single representation in DWARF. For gdb's purposes it is fine if this representation requires some special Rust-specific code, for example knowing what names to insert into what scopes.

This is related to issue #32920, in that maybe a tagged variant would also help this situation.

[michaelwoerister]

We should be able to set the name of the DW_TAG_enumeration_type entry to something like __<enum-name>_discriminant. I'll need to look up though how we represent C-like enums (probably just the DW_TAG_enumeration_type without the ``DW_TAG_union_type`) and if that needs special attention then.

[tromey]

I may be fixing this as a side effect of work on #32920