'()' should be DW_TAG_unspecified_type

I tried this code:

fn main() {
    let x = ();
}

I compiled with -g and examined the output with readelf -wi. I see this:

 <1><5e>: Abbrev Number: 4 (DW_TAG_typedef)
    <5f>   DW_AT_name        : ()
    <62>   DW_AT_decl_file   : 2
    <63>   DW_AT_decl_line   : 0
    <64>   DW_AT_type        : <0x6d>
...
 <1><6d>: Abbrev Number: 6 (DW_TAG_structure_type)
    <6e>   DW_AT_byte_size   : 0

In addition to the typedef issue (see #1740), I think it would be better for () to use DW_TAG_unspecified_type. This is what rustc does now (it used to use a 0-width integer type).

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git gccrs, commit 2f9f77f9dd6330fe60554400133d7217f78afa43