Skip new parser validation when skipping function bodies

[bnbarham]

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.

[bnbarham]

FWIW this can easily fail locally for anyone using an asserts compiler. But SwiftPM tests also fail without this, it's just that they aren't run on CI due to SWIFTCI_DISABLE_SDK_DEPENDENT_TESTS being set. The issue there is that when we're compiling a project, we may end up creating a job to output the swiftmodule skipping non-inlinable function bodies. Then that fails due to the verification check with a potentially different diagnostic to the old swift parser. This fails in the test since it's expecting a particular message:

error: consecutive statements on a line must be separated by newline or ';'

vs

error: consecutive statements on a line must be separated by ';'

[hamishknight]

LGTM, would it be worth adding a test?

[bnbarham]

@swift-ci please test