Move make_temp_filename to common/pathstuff.c

Move make_temp_filename to common/pathstuff.c

Currently make_temp_filename is a function local to
write_psymtabs_to_index.  This patch moves it to pathstuff.c so that
it can be used from other places in gdb.

gdb/ChangeLog
2018-10-27  Tom Tromey  <tom@tromey.com>

	* dwarf-index-write.c (write_psymtabs_to_index): Move
	make_temp_filename to common/pathstuff.c.
	* common/pathstuff.h (make_temp_filename): Declare.
	* common/pathstuff.c (make_temp_filename): New function, moved
	from dwarf-index-write.c.