Change build_address_symbolic to return std::string

Change build_address_symbolic to return std::string

This changes two out parameters of build_address_symbolic to be
std::string, and updates the callers.  This allows removing some
cleanups.

This patch also moves the declaration of build_address_symbolic out of
defs.h.  I think that many things in defs.h should be elsewhere
instead.  In this case, I moved the declaration to valprint.h, becuase
there is no "printcmd.h" -- but perhaps it would be better to
introduce that instead.

Tested by the buildbot.

gdb/ChangeLog
2018-06-07  Tom Tromey  <tom@tromey.com>

	* valprint.h (build_address_symbolic): Declare.
	* printcmd.c (print_address_symbolic): Update.
	(build_address_symbolic): Change "name" and "filename" to
	std::string.
	* disasm.c (gdb_pretty_print_disassembler::pretty_print_insn):
	Update.
	* defs.h (build_address_symbolic): Remove declaration.