Each macro expansion buffer was getting parsed twice: once by
ParseSourceFileRequest (which is used by unqualified name lookup) and
once to parse the expression when type-checking the expanded macro.
This meant that the same code had two ASTs. Hilarity ensures.
Stop directly invoking the parser on macro-expanded code. Instead, go
through ParseSourceFileRequest *as is always the right way*, and dig
out the expression we want.
The swift-syntax diagnostic system always treats Fix-Its as separate
notes, which are never attached to the primary diagnostic. Embrace this
module in the mapping over to the existing C++ diagnostic engine.
"Relink" the folded syntax node back into the primary syntax node. When
we do this, we get consistent source locations that do not require any
adjustment.
Test this by adding a Fix-It to the silly AddBlocker macro, replacing
the `+` with a `-`.
