gdb: pass objfile_per_bfd_storage instead of objfile to partial_symtab

gdb: pass objfile_per_bfd_storage instead of objfile to partial_symtab

Since partial_symtab is supposed to be objfile-independent (since series
[1]), I think it would make sense for partial_symtab to not take an
objfile as a parameter in its constructor.

This patch replaces that parameter with an objfile_per_bfd_storage
parameter.

The objfile is used for two things:

 - to get the objfile_name, for debug messages.  We can get that name
   from the bfd instead.
 - to intern the partial symtab filename.  Even though it goes through
   an objfile method, the request is actually forwarded to the
   underlying objfile_per_bfd_storage.  So we can ask the new
   objfile_per_bfd_storage instead.

In order to get a reference to the BFD from the objfile_per_bfd_storage,
the BFD is saved in the objfile_per_bfd_storage object.

[1] https://sourceware.org/pipermail/gdb-patches/2021-February/176625.html

gdb/ChangeLog:

	* psympriv.h (struct partial_symtab) <partial_symtab>: Change
	objfile parameter for objfile_per_bfd_storage, adjust callers.
	(struct standard_psymtab) <standard_psymtab>: Likewise.
	(struct legacy_psymtab) <legacy_psymtab>: Likewise.
	* psymtab.c (partial_symtab::partial_symtab): Likewise.
	* ctfread.c (struct ctf_psymtab): Likewise.
	* dwarf2/read.h (struct dwarf2_psymtab): Likewise.
	* dwarf2/read.c (struct dwarf2_include_psymtab): Likewise.
	(dwarf2_create_include_psymtab): Likewise.
	* objfiles.h (struct objfile_per_bfd_storage)
	<objfile_per_bfd_storage>: Add bfd parameter, adjust callers.
	<get_bfd>: New method.
	<m_bfd>: New field.
	* objfiles.c (get_objfile_bfd_data): Adjust.

Change-Id: I2ed3ab5d2e6f27d034bd4dc26ae2fae7b0b8a2b9