Eliminate BinaryTypeTraitExpr

There's nothing special about type traits accepting two arguments.

This commit eliminates BinaryTypeTraitExpr and switches all related handling
over to TypeTraitExpr.

Also fixes a CodeGen failure with variadic type traits appearing in a
non-constant expression.

The BTT/TT prefix and evaluation code is retained as-is for now but will soon
be further cleaned up.

This is part of the ongoing work to unify type traits.