This would otherwise result in false positives, since if the old parser
skipping a body with errors would cause a verification failure.
Don't perform round trip validation either, since we'll presumbly still
hit parsing the full file when not skipping bodies - there's no point
running it twice.
Resolves rdar://111032175.
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.
Introduce the experimental feature `ParserDiagnostics`, which emits
diagnostics from the new Swift parser *first* for a source file. If
that produces any errors, we suppress any diagnostics emitted from the
C++ parser.
