Fix name mismatch for fortran.

Fix name mismatch for fortran.

Function name in afdo_string_table is step3d_t_tile.
but DECL_ASSEMBLER_NAME (edge->callee->decl))) gets
__step3d_t_mod_MOD_step3d_t_tile, Looks like the prefix is not in the
debug string table.
The patch uses
afdo_string_table->get_index_by_decl (edge->callee->decl) instead.

gcc/ChangeLog:

	PR gcov-profile/118508
	* auto-profile.cc
	(autofdo_source_profile::get_callsite_total_count): Fix name
	mismatch for fortran.