Remove `CanImportExprSyntax` and provide API to interpret an `ExprSyntax` as a `VersionTupleSyntax`

[ahoppen]

canImport inside of #if should get parsed the same as in a normal expression position, eliminating the need for CanImportExprSyntax (sparked by link). This means that the parser won’t be able to generate diagnostics for invalid version numbers inside canImport anymore.

Instead, we should have a function on ExprSyntax that interprets the expression (which would be a FloatingLiteralExprSyntax for eg. 1.2) as a VersionTupleSyntax. If the ExprSyntax doesn’t represent a version, that function should return a list of diagnostics. SwiftIfConfig would then call that getter and would be responsible for reporting any errors that occur from interpreting the ExprSyntax of the _version argument as VersionTupleSyntax.

[ahoppen]

Tracked in Apple’s issue tracker as rdar://113236781