fully qualifed IR names causes problems of mismatched definitions

[Basile B.]

This is a bug report.

The context is that I have bindings that I want to translate with 
more friendly names and in a less C fashion. There's many to do 
so I wish to proceed gradually, which leads to an intermediate 
situation where the C definitions exists in two different modules.

LDC does not like that because it seems that IR names for 
extern(C) definitions are fully qualified.

Minimal reproduction on linux, file is named _test.sh_:

```bash
echo "module a;
extern(C):
struct S{}
S* create(){return null;} " > a.d

echo "module b;
extern(C):
struct S;
S* create();
void t(){auto s = create();}
" >b.d

echo "module c;
extern(C):
struct S;
S* create();
void main(){auto s = create();}
" > c.d

echo "================DMD================"
dmd a.d b.d c.d
echo "dmd is happy with that"
echo "================GDC================"
gdc a.d b.d c.d
echo "gdc is happy with that"
echo "================LDC================"
ldc2 a.d b.d c.d
```

which results in:

```console
[xxxx at pc extern_c_irtype_bug]$ bash test.sh
================DMD================
dmd is happy with that
================GDC================
gdc is happy with that
================LDC================
b.d(4): Error: Function type does not match previously declared 
function with the same mangled name: create
b.d(4):        Previous IR type: %c.S* ()
b.d(4):        New IR type:      %b.S* ()
```

Maybe that the IR name for extern(C) types should only include 
the last identifer ?