Constify main_name

This patch constifies the return type of main_name.  There is a
comment indicating that this wasn't possible at some point in the
past, but whatever the barrier was, it is gone now.

Tested by rebuilding.

gdb/ChangeLog
2019-07-18  Tom Tromey  <tromey@adacore.com>

	* symtab.c (main_name): Constify return type.
	* symfile.c (set_initial_language): Update.
	* symtab.h (main_name): Constify return type.