Turn start_psymtab_common into a constructor

Turn start_psymtab_common into a constructor

This turns start_psymtab_common into a constructor, and then changes
the callers to use "new" directly.  This completes the psymtab
allocation transition -- now it is possible for symbol readers to
subclass struct partial_symtab.

gdb/ChangeLog
2020-01-26  Tom Tromey  <tom@tromey.com>

	* xcoffread.c (xcoff_start_psymtab): Use new.
	* psymtab.c (partial_symtab::partial_symtab): New constructor,
	renamed from start_psymtab_common.
	* psympriv.h (struct partial_symtab): Add new constructor.
	(start_psymtab_common): Don't declare.
	* mdebugread.c (parse_partial_symbols): Use new.
	* dwarf2read.c (create_partial_symtab): Use new.
	* dbxread.c (start_psymtab): Use new.
	* ctfread.c (create_partial_symtab): Use new.

Change-Id: I5a0217bcb52bcfa442559771954bb66bd9ccbf02