Do not allocate psymtabs via psymtab_storage

Do not allocate psymtabs via psymtab_storage

Currently, partial symbol tables are allocated by a method in
psymtab_storage.  However, eventually we want to subclass partial
symtabs in the symbol readers, so the calls to "new" will have to
happen there.  This patch is a first step, moving the allocation from
psymtab_storage and into allocate_psymtab.

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

	* psymtab.h (class psymtab_storage) <install_psymtab>: Rename from
	allocate_psymtab.  Update documentation.
	* psymtab.c (psymtab_storage::install_psymtab): Rename from
	allocate_psymtab.  Do not use new.
	(allocate_psymtab): Use new.  Update.

Change-Id: Iba6a9bf3ee1e78062fdb9f007c3010f826f64bc8