This eliminates a pile of now-dead code in:
* The type checker, where we no longer have special cases for bridging conversions
* The expression ASTs, where we no longer need to distinguish bridging collection up/down casts
* SILGen, which no longer uses
Still to come is the removal of the
_(set|dictionary)Bridge(From|To)ObjectiveC(Conditional)? entrypoints
from the standard library. They're still used by some tests.
Implements part of SE-0110. Single argument in closures will not be accepted if
there exists explicit type with a number of arguments that's not 1.
```swift
let f: (Int, Int) -> Void = { x in } // this is now an error
```
Note there's a second part of SE-0110 which could be considered additive,
which says one must add an extra pair of parens to specify a single arugment
type that is a tuple:
```swift
let g ((Int, Int)) -> Void = { y in } // y should have type (Int, Int)
```
This patch does not implement that part.
