[C++] Avoid exposing internal details in aka types

[C++] Avoid exposing internal details in aka types

This patch extends r276951 to work for C++ too.

2019-10-22  Richard Sandiford  <richard.sandiford@arm.com>

gcc/cp/
	* cp-tree.h (STF_USER_VISIBLE): New constant.
	(strip_typedefs, strip_typedefs_expr): Take a flags argument.
	* tree.c (strip_typedefs, strip_typedefs_expr): Likewise,
	updating mutual calls accordingly.  When STF_USER_VISIBLE is true,
	only look through typedefs if user_facing_original_type_p.
	* error.c (dump_template_bindings, type_to_string): Pass
	STF_USER_VISIBLE to strip_typedefs.
	(dump_type): Likewise, unless pp_c_flag_gnu_v3 is set.

gcc/testsuite/
	* g++.dg/diagnostic/aka5.h: New test.
	* g++.dg/diagnostic/aka5a.C: Likewise.
	* g++.dg/diagnostic/aka5b.C: Likewise.
	* g++.target/aarch64/diag_aka_1.C: Likewise.

From-SVN: r277281