[SwiftParser] Experimental feature flags for _borrow and _move

[rintaro]

_borrow and _move should be parsed as an ownership operator only when a feature flag is enabled.

[rintaro]

swiftlang/swift#80046
@swift-ci Please test

[rintaro]

swiftlang/swift#80046
@swift-ci Please test

[rintaro]

swiftlang/swift#80046
@swift-ci Please test

[rintaro]

swiftlang/swift#80046
@swift-ci Please test

[rintaro]

swiftlang/swift#80046
@swift-ci Please test

[rintaro]

swiftlang/swift#80046
@swift-ci Please test Windows

[ahoppen]

Could you make these preconditions?

swift-syntax/Sources/SwiftSyntax/Assert.swift

Lines 13 to 22 in ee01462

	/// How to choose `assert` vs. `precondition`:
	/// - Wherever possible, it is preferable to emit a diagnostic instead of
	/// using `precondition`. This way the parser won't crash if the condition is
	/// violated.
	/// - If you think the diagnostic added above should never be emitted, it is
	/// fine to also emit an `assertionFailure` in debug builds to make it easier
	/// to debug the unexpected diagnostic.
	/// - If in doubt always use `precondition`
	/// - `assert` should only be used if checking the assertion has a non-trivial
	/// cost and provides little benefit in terms of safety in release builds.

[rintaro]

I'm not really sure it worth, although it trivial, The branch is a result of the subject expression, where the experimentalFeatures is handled, which is close enough for not checking seriously. Also even if somehow we enter this branch without oldOwnershipOperatorSpellings feature in release builds, we still have atContextualExpressionModifier() condtion check below so the check is just to make sure the condition check in ExpressionModifierKeyword.init(lexeme:) is working.

FWIW, looking at the existing code, do expression uses precodition but then statement uses assert.