This migrates anything using the following pattern:
```swift
if let optX = try? someOptional(),
let x = optX,
...
```
to this:
```swift
if let x = try? someOptional(),
...
```
In Swift3, shadowning type(of:) was impossible, because it was a parser
magic. In Swift4, type(of:) is resolved as normal function in stdlib so
it can be shadowed. 'TypeOfMigratorPass' was a targeted migrator pass
that prepend 'Swift.' to 'type(of:)' so that it refers Swift.type(of:)
stdlib builtin function.
We used to assert migration scripts exist. This patch further
decouples these scripts and the compiler by treating missing scripts
as a regular compiler error.
Related: rdar://40538097
The SyntacticMigratorPass is getting Too Big To Fail and covers
multiple migrations. There was already an early exit to not run
the pass if APIDigesterDataStorePath wasn't supplied, so SE-0110
tuple splat fix-its weren't getting run. This manifested in Migrator
tests not printing migrated contents on Linux.
New: ASTMigratorPass
Renamed: SyntacticMigratorPass -> APIDiffMigratorPass
New: TupleSplatMigratorPass
These implementations are entirely hidden and can only be invoked
by a swift::migrator function.
rdar://problem/32025974
