Most of AST, Parse, and Sema deal with FileUnits regularly, but SIL
and IRGen certainly don't. Split FileUnit out into its own header to
cut down on recompilation times when something changes.
No functionality change.
We were trying to get fancy rewriting cases where multiple 'if let' clauses were
used to unwrap a double optional, but in testing that turned out to be too
unreliable. Now we simply detect any 'try?' expressions whose behavior will change,
and add an explicit 'as OldType' to the expression, in order to preserve the original
behavior.
This migrates anything using the following pattern:
```swift
if let optX = try? someOptional(),
let x = optX,
...
```
to this:
```swift
if let x = try? someOptional(),
...
```
