Auto merge of #96316 - michaelwoerister:debuginfo-fix-unit-msvc, r=wesleywiser

debuginfo: Emit ZST struct debuginfo for unit type when CPP-like debuginfo is enabled

As already discovered in https://github.com/rust-lang/rust/commit/24a728a8eb4832568509eb757c2374934a76cb98, PDB does not play well with custom basic types. This PR extends to the fix to `()`: Instead of a custom basic type, we treat it like an empty tuple (i.e. it is described as a struct which happens to have no fields).

Before this change anything with a `()` in it would cause trouble, which is especially bad for `*const ()` and `*mut ()` which are often used for opaque pointers. E.g. the test case added in this PR would look like:
```
0:000>  dx _ref
Error: Unable to bind name '_ref'
0:000>  dx _ptr
Error: Unable to bind name '_ptr'
0:000>  dx _local
Error: Unable to bind name '_local'
0:000>  dx _field,d
_field,d         [Type: unit_type::_TypeContainingUnitField]
    [+0x008] _a               : 123 [Type: unsigned int]
    [+0x000] _unit            : Unexpected failure to dereference object
    [+0x000] _b               : 456 [Type: unsigned __int64]
0:000>  dx ((__int64 *)_ptr),x
Error: Unable to bind name '_ptr'
```

With the PR it produces the expected output:
```
0:000>  dx _ref
_ref             : 0x7ff6f2012230 : () [Type: tuple$<> *]
0:000>  dx _ptr
_ptr             : 0x7e8ddffc20 : () [Type: tuple$<> *]
0:000>  dx _local
_local           : () [Type: tuple$<>]
0:000>  dx _field,d
_field,d         [Type: unit_type::_TypeContainingUnitField]
    [+0x008] _a               : 123 [Type: unsigned int]
    [+0x000] _unit            : () [Type: tuple$<>]
    [+0x000] _b               : 456 [Type: unsigned __int64]
0:000>  dx ((__int64 *)_ptr),x
((__int64 *)_ptr),x : 0x7e8ddffc20 : 0x1122334455667788 [Type: __int64 *]
```

r? `@wesleywiser`

0 files changed, 0 insertions, 0 deletions