DWARF: `no_mangle` items are given mangled `linkage_name`s and namespace scopes

[m4b]

This technically subsumes:

#33172

The problem is:

1. any item marked no_mangle is given a mangled linkage_name
2. any item marked no_mangle is given a dwarf namespace scope

So basically, it's not correct at all.

On linux gdb this manifests in various ways, usually reporting the "symbol is in a file with no debugging information", and on lldb (tested on osx) the only way to disassemble a function, for example, is to namespace the mangled function like disass -n main::unmangled_function, which is obviously counter-intuitive.

According to the dwarf standard:

If the object file name for an entity is the same as the name in
the source program, then a linkage name attribute is omitted.

So I think the right thing to do here is not omit a linkage_name at all.

The PR in #46457 fixes this for statics; the change for functions and any other relevant no_mangle items should be relatively straightforward after.

[steveklabnik]

Triage: it would be nice to have a way to reproduce this issue in this ticket.

[tmiasko]

Triage: For no_mangle items the linkage_name is included but value is correct, i.e., not mangled.