swift234

Introduce lazy computation of a BoundGenericType's substitutions.

Teach a BoundGenericType to compute its own substitutions, which
allows AST clients to create new bound generic types without the aid
of the type checker. 

This eliminates the TypeChecker::validateTypeSimple() abomination as
well as the need for the BoundGenericType AST validation step. There
is still more cleanup to do in this area.

Note that BoundGenericType::getSubstitutions() now accepts a module
parameter, which is the place from which we will look for
conformances. This is a baby step toward properly modeling the
conformances as part of the bound generic type, and is nowhere near
complete.


Swift SVN r8193