swift234

Use different TypeInfo instances for generic types whenever
anything major about IRGen for that type would change,
including the IR type of the value's components.

This generally makes the IR types we use for generic types
more precise, which means we also finally need to properly
remap loadable dependent types when they appear as the
result of a generic function call.  This basically only
comes up with class-bounded archetypes, which is why we
haven't run into it a whole lot before.

This all makes a whole lot of things in IRGen involving
generic structs a lot happier and, in particular, fixes
rdar://17986446, the "Unmanaged is horribly busted" bug.

I needed to carve out a big ol' exception for enums, which
apparently cannot be reliably specialized with substituted
types in all cases because of problems with multi-payload
strategies.  The exception reinstates the current, incorrect
rule of using a shared implementation whenever the
pattern has a fixed size.  This is rooted in a bunch of
deep, known, and tracked problems that I don't have time
to solve.

Swift SVN r21348