Only use the per-BFD object to write a DWARF index

The DWARF index does not need access to the objfile or per-objfile
objects when writing -- it's entirely based on the objfile-independent
per-BFD data.

This patch implements this idea by changing the entire API to only be
passed the per-BFD object.  This simplifies some lifetime reasoning
for the next patch.

This patch removes some code that ensures that the BFD came from a
file.  It seems to me that checking for the existence of a build-id is
good enough for the index cache.

gdb/dwarf2/index-write.h

@@ -33,7 +33,7 @@
    same, but for the dwz file's index.  */
 
 extern void write_dwarf_index
-  (dwarf2_per_objfile *per_objfile, const char *dir, const char *basename,
+  (dwarf2_per_bfd *per_bfd, const char *dir, const char *basename,
    const char *dwz_basename, dw_index_kind index_kind);
 
 #endif /* DWARF_INDEX_WRITE_H */