[Sema] Add fix-it to import RegexBuilder
For code such as the following:
```
let r = Regex {
/abc/
}
```
If RegexBuilder has not been imported, emit a
specialized diagnostic and fix-it to add
`import RegexBuilder` to the file.
Unfortunately we're currently prevented from
emitting the specialized diagnostic in cases where
the builder contains references to RegexBuilder
types, such as:
```
let r = Regex {
Capture {
/abc/
}
}
```
This is due to the fact that we bail from CSGen
due to the reference to `Capture` being turned
into an `ErrorExpr`. We ought to be able to
handle solving in the presence of such errors, but
for now I'm leaving it as future work.
rdar://93176036
