swift234

When gathering "linked" expressions to determine a favored constraint, consider top-level literals to be fair game.

This unblocks standard library work by preventing us from going exponential when extending existing struct types to have failable initializers. (rdar://problem/20336356)
On my laptop, it also results in a 7% end-to-end improvement in the time it takes to run our unit tests under DebugAssert (previously: 591.63s, now: 552.64s). Though, as usual, YMMV.

Swift SVN r27156