[llvm] DWARF: Emit `DW_AT_artificial` on artificial globals (PR #93118)

[Michael Buch via llvm-commits]

Michael137 wrote:

> Perhaps other folks can chime in as to whether it's worth putting a Flags attribute on DIGlobalVariable

Agreed it would be nice to attach `DIFlagArtificial` on both the local and global `DIVariable`s

That would then also help for cases where the variable is artificial but the type is not. E.g., in this contrived example:
```
#include <utility>

struct Pair {
    int x, y;
};

auto get() { return std::pair{1, 2}; }

auto [g_x, g_y] = get();

int main() {
    auto [l_x, l_y] = get();

    return g_x + g_y + l_x + l_y;
}
```

```
0x000007cc:   DW_TAG_variable
                DW_AT_name      ("g_x")
                DW_AT_type      (0x000007d7 "int &&")
                DW_AT_external  (true)
                DW_AT_decl_file ("/Users/michaelbuch/artificial.cpp")
                DW_AT_decl_line (9)
                DW_AT_location  (DW_OP_addrx 0x1)

0x00000864:     DW_TAG_variable
                  DW_AT_location        (DW_OP_breg31 WSP+24)
                  DW_AT_name    ("l_x")
                  DW_AT_type    (0x000007d7 "int &&")
                  DW_AT_artificial      (true)
```


just my 2c

https://github.com/llvm/llvm-project/pull/93118