gdb/ctf: pass partial symtab's filename to buildsym_compunit

gdb/ctf: pass partial symtab's filename to buildsym_compunit

I noticed that the CTF symbol reader passes the objfile's name to all
buildsym_compunit instances it creates.  The result is that all
compunit_symtabs created have the same name, that of the objfile:

    { objfile /tmp/babeltrace-ctf/src/lib/.libs/libbabeltrace2.so.0.0.0 ((struct objfile *) 0x613000005d00)
      { ((struct compunit_symtab *) 0x621000286760)
        debugformat ctf
        producer (null)
        name libbabeltrace2.so.0.0.0
        dirname (null)
        blockvector ((struct blockvector *) 0x6210003911d0)
        user ((struct compunit_symtab *) (null))
            { symtab /tmp/babeltrace-ctf/src/lib/.libs/libbabeltrace2.so.0.0.0 ((struct symtab *) 0x6210003911f0)
              fullname (null)
              linetable ((struct linetable *) 0x0)
            }
      }
      { ((struct compunit_symtab *) 0x621000275c10)
        debugformat ctf
        producer (null)
        name libbabeltrace2.so.0.0.0
        dirname (null)
        blockvector ((struct blockvector *) 0x621000286710)
        user ((struct compunit_symtab *) (null))
            { symtab /tmp/babeltrace-ctf/src/lib/.libs/libbabeltrace2.so.0.0.0 ((struct symtab *) 0x621000286730)
              fullname (null)
              linetable ((struct linetable *) 0x0)
            }
      }

Notice the two "name libbabeltrace2.so.0.0.0".

Change it to pass the partial_symtab's filename instead.  The output
becomes:

    { objfile /tmp/babeltrace-ctf/src/lib/.libs/libbabeltrace2.so.0.0.0 ((struct objfile *) 0x613000005d00)
      { ((struct compunit_symtab *) 0x621000295610)
        debugformat ctf
        producer (null)
        name libbabeltrace2.so.0.0.0
        dirname (null)
        blockvector ((struct blockvector *) 0x6210003a15d0)
        user ((struct compunit_symtab *) (null))
            { symtab /tmp/babeltrace-ctf/src/lib/.libs/libbabeltrace2.so.0.0.0 ((struct symtab *) 0x6210003a15f0)
              fullname (null)
              linetable ((struct linetable *) 0x0)
            }
      }
      { ((struct compunit_symtab *) 0x621000288700)
        debugformat ctf
        producer (null)
        name current-thread.c
        dirname (null)
        blockvector ((struct blockvector *) 0x6210002955c0)
        user ((struct compunit_symtab *) (null))
            { symtab /home/simark/src/babeltrace/src/lib/current-thread.c ((struct symtab *) 0x6210002955e0)
              fullname (null)
              linetable ((struct linetable *) 0x0)
            }
      }

Note that the first compunit_symtab still has libbabeltrace2.so.0.0.0 as
its name.  This is because the CTF symbol reader really creates a
partial symtab named like this.  It appears to be because the debug info
contains information that has been factored out of all CUs and is at the
"top-level" of the objfile, outside any real CU.  So it creates a
partial symtab and an artificial CU that's named after the objfile.

Change-Id: I576316bab2a3668adf87b4e6cebda900a8159b1b