Multiple extern fn with same name should not error out

[nagisa]

See #16403 (comment) and that issue in general.

Currently for each extern fn we generate a rust ABI function containing function body and a C ABI trampoline to the rust function. However we do not mangle the rust ABI function sufficiently, therefore this code does not compile:

fn main() {
    { extern fn a() {this} };
    { extern fn a() {that} };
}

[apasel422]

Closing as a duplicate of #26201.