RFC (attributes): PATCH for c++/50800 to set affects_type_identity for may_alias

[Jason Merrill]

My earlier patch for 50800 fixed the ICE by consistently stripping 
non-mangled attributes from template arguments, and mangling those that 
affect type identity.  At the C++ meeting this week someone pointed out 
to me that this is a real problem for x86 vector code, which relies on 
may_alias semantics: if may_alias is stripped from __m128, users can't 
use templates with vectors.

So, it seems that the solution is to mangle may_alias by saying that it 
affects type identity.  But since we still want to be able to convert 
back and forth, I thought that it would make sense to treat the 
may_alias version of a type as a variant, rather than a new distinct 
type.  So the first patch creates a new category of attributes that are 
treated as type variants.

An alternative patch just sets affects_type_identity and adjusts the C++ 
front end to allow conversion between pointers to add or discard may_alias.

Thoughts?

Tested x86_64-pc-linux-gnu.
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