c++: Constrain visibility for CNTTPs with internal types [PR118849]

c++: Constrain visibility for CNTTPs with internal types [PR118849]

While looking into PR118846 I noticed that we don't currently constrain
the linkage of functions involving CNTTPs of internal-linkage types.  It
seems to me that this would be sensible to do.

	PR c++/118849

gcc/cp/ChangeLog:

	* decl2.cc (min_vis_expr_r): Constrain visibility according to
	the type of decl_constant_var_p decls.

gcc/testsuite/ChangeLog:

	* g++.dg/template/linkage6.C: New test.

Signed-off-by: Nathaniel Shead <nathanieloshead@gmail.com>