Auto merge of #46457 - m4b:no_mangle_static, r=michaelwoerister

Don't set the linkage_name for static variables

For `no_mangle` statics:

1. Linkage_name no longer set
2. The static variable also no longer has a dwarf namespace scope

This matches C++ output, which does not set the linkage_name and is not scoped:

e.g. c++:

```
0x000000b6:   DW_TAG_base_type [8]
                DW_AT_name [DW_FORM_strp]       ( .debug_str[0x00000077] = "long int")
                DW_AT_encoding [DW_FORM_data1]  (DW_ATE_signed)
                DW_AT_byte_size [DW_FORM_data1] (0x08)

0x000000bd:   DW_TAG_variable [9]
                DW_AT_name [DW_FORM_strp]       ( .debug_str[0x00000053] = "TEST")
                DW_AT_type [DW_FORM_ref4]       (cu + 0x0048 => {0x00000048})
                DW_AT_external [DW_FORM_flag_present]   (true)
                DW_AT_decl_file [DW_FORM_data1] ("/home/m4b/tmp/bad_debug/test.cpp")
                DW_AT_decl_line [DW_FORM_data1] (14)
                DW_AT_location [DW_FORM_exprloc]        (<0x9> 03 40 10 20 00 00 00 00 00 )

0x000000d2:   DW_TAG_namespace [2] *
                DW_AT_name [DW_FORM_strp]       ( .debug_str[0x0000009d] = "std")

```

and (now) Rust:

```
0x0000002a:   DW_TAG_variable [2]
                DW_AT_name [DW_FORM_strp]       ( .debug_str[0x00000046] = "TEST")
                DW_AT_type [DW_FORM_ref4]       (cu + 0x0045 => {0x00000045})
                DW_AT_external [DW_FORM_flag_present]   (true)
                DW_AT_decl_file [DW_FORM_data1] ("/tmp/test.rs")
                DW_AT_decl_line [DW_FORM_data1] (8)
                DW_AT_alignment [DW_FORM_udata] (1)
                DW_AT_location [DW_FORM_exprloc]        (<0x9> 03 c0 4d 06 00 00 00 00 00 )

0x00000040:   DW_TAG_namespace [3] *
                DW_AT_name [DW_FORM_strp]       ( .debug_str[0x0000004b] = "test")
```

diff --cc src/librustc_trans/debuginfo/metadata.rs
index 12cd874f86887ee54ae287ad717fb4241b00a994,6602a31b5c5b76209ed5036623590b5e514cecbe..8e924e3f0ad925192723966af1e6610dfbbdd816
--- 1/src/librustc_trans/debuginfo/metadata.rs
--- 2/src/librustc_trans/debuginfo/metadata.rs
+++ b/src/librustc_trans/debuginfo/metadata.rs
@@@ -1656,13 -1658,16 +1658,16 @@@ pub fn create_global_var_metadata(cx: &
      };
  
      let is_local_to_unit = is_node_local_to_unit(cx, node_id);
 -    let variable_type = common::def_ty(cx.tcx(), node_def_id, Substs::empty());
 +    let variable_type = Instance::mono(cx.tcx(), node_def_id).ty(cx.tcx());
      let type_metadata = type_metadata(cx, variable_type, span);
      let var_name = tcx.item_name(node_def_id).to_string();
-     let linkage_name = mangled_name_of_item(cx, node_def_id, "");
- 
      let var_name = CString::new(var_name).unwrap();
-     let linkage_name = CString::new(linkage_name).unwrap();
+     let linkage_name = if no_mangle {
+         None
+     } else {
+         let linkage_name = mangled_name_of_item(cx, node_def_id, "");
+         Some(CString::new(linkage_name).unwrap())
+     };
  
      let global_align = cx.align_of(variable_type);