Lower metatype types as @thin or @thick based on whether the type is static and whether the abstraction pattern allows for a thin metatype. Add a '@thick' attribute and require SIL metatypes to always be annotated with either '@thin' or '@thick' to distinguish them from unlowered metatypes.
Swift SVN r11525
Reuse John's abstraction thunking machinery in SILGenPoly to emit the abstraction change from a protocol requirement to a concrete witness. There are potentially two abstraction changes necessary; if a witness is generic, we need to reabstract again from the concrete substituted type of the witness to the generic witness function's original signature. This currently leads to a bunch of extra temporaries in cases where an argument or return gets unabstracted to a loadable value then reabstracted to a generic parameter, but optimizations should be able to clean this up. Protocol witnesses also have additional potential abstraction changes in their 'self' parameter: the 'self' parameter of the protocol requirement is always considered @inout, but class 'self' parameters are not; also, an operator requirement can be satisfied by a free function, in which case 'self' is discarded.
Also, fix a bug in return value thunking where, if the thunk could reuse its @out parameter as the @out parameter of the underlying function, we would not disable the cleanup we install on the result value, leading to the result getting overreleased.
Swift SVN r11245
