Teach IRGen how to emit thunks for SpecializeInsts that aren't immediately called and actually get used as values. This allows generic function instance to get passed around as values (again), and is a step along the way to making closures in generic contexts work (so we can specialize the local function, then partially apply its specialized context).
This doesn't work yet if we specialize to local archetypes--SIL needs to learn that we need a [thick]-typed thunk for local archetype specializations, in order to pack the metadata and wtables for the local type variables.
Swift SVN r5083
Create a new FallthroughStmt, which transfers control from a 'case' or 'default' block to the next 'case' or 'default' block within a switch. Implement parsing and sema for FallthroughStmt, which syntactically consists of a single 'fallthrough' keyword. Sema verifies that 'fallthrough' actually appears inside a switch statement and that there is a following case or default block to pass control to.
SILGen/IRGen support forthcoming.
Swift SVN r4653
