Address Tom Tromey's comments on the CTF reader.

Address Tom Tromey's comments on the CTF reader.

 * Use the type-safe registry for ctf_file_key;
 * Drop "typedef" when defining "struct ctf_context";
 * Use ANOFFSET with SECT_OFF_TEXT to get the text base address;

gdb/ChangeLog
2019-12-20  Weimin Pan  <weimin.pan@oracle.com>
        * ctfread.c (ctf_file_key): Change type to objfile_key.
        (struct ctf_context): Remove typedef.
        (get_objfile_text_range): Use ANOFFSET to get text base.