Summary:
Phab:D907 introduced SourceText for a number of data types, by replacing
FastString with (SourceText,FastString). Since this has an Outputable
instance, no warnings are generated when ppr is called on it, but
unexpected output is generated. See Phab:D1096 for an example of this.

Replace the (SourceText,FastString) tuples with a new data type,
```lang=hs
data StringLiteral = StringLiteral SourceText FastString
```

Update haddock submodule accordingly

Test Plan: ./validate

Reviewers: hvr, austin, rwbarton, trofi, bgamari

Reviewed By: trofi, bgamari

Subscribers: thomie, trofi, rwbarton, mpickering

Differential Revision: https://phabricator.haskell.org/D1101

GHC Trac Issues: #10692