Two compute_and_set_names simplifications

This patch simplifies compute_and_set_names in a couple of ways.

First, it changes one spot to use obstack_strndup, which is
equivalent, but more concise.

Second, the function ends with two calls to symbol_set_demangled_name.
This can be simplified to a single call.

gdb/ChangeLog
2020-02-19  Tom Tromey  <tom@tromey.com>

	* symtab.c (general_symbol_info::compute_and_set_names): Use
	obstack_strndup.  Simplify call to symbol_set_demangled_name.