* [SwiftSyntax] Add SyntaxRewriter.visitAny(_:)
This function, when overridden, allows Syntax rewriters to perform custom dynamic visitation behavior. If a user wanted to, say, store a series of transformations accessible by metatype, they can override visitAny and do their own runtime dispatch.
If a non-nil result is returned from visitAny, the original specialized visitors are skipped.
Based on the feedbacks from our early adopters, separating syntax tree analysis
with transformation is a common pattern. Thus, we introduce a read-only
syntax tree visitor to help the analysis phase. This visitor never alters the
content of a tree being visited, in contrast to SyntaxRewriter which always does.
* Create Swift libSyntax API
This patch is an initial implementation of the Swift libSyntax API. It
aims to provide all features of the C++ API but exposed to Swift.
It currently resides in SwiftExperimental and will likely exist in a
molten state for a while.
* Only build SwiftSyntax on macOS
