HsTyLit now has SourceText, and small other tweaks

Summary:
  HsTyLit now has SourceText

  Small documentation tweaks

  Ensure that the parser always keeps HsSCC and HsTickPragma values, to
  be ignored in the desugar phase if not needed

  Bringing in SourceText for pragmas

Test Plan: ./validate

Reviewers: hvr,austin

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GHC Trac Issues: