Use add_partial_symbol in load_partial_dies

Use add_partial_symbol in load_partial_dies

An earlier patch added the add_partial_symbol helper function to
dwarf2/read.c.  However, a couple of calls to add_psymbol_to_list were
left in place.  It turns out that these calls slow down partial symbol
reading, because they still go via the path that tries to needlessly
demangle already-demangled names.

This patch improves the performance of partial symbol reading by
changing this code to use add_partial_symbol instead.

The run previous to this had times of (see the first patch in the
series for an explanation):

gdb    1.64
libxul 1.99
Ada    2.47

This patch improves the times to:

gdb    1.47
libxul 1.89
Ada    2.39

gdb/ChangeLog
2020-05-27  Tom Tromey  <tromey@adacore.com>

	* dwarf2/read.c (load_partial_dies): Use add_partial_symbol.