SwiftSyntax: Factor out an API to decode serialized SourceFileSyntax. NFC (#14164)

Since our recent integration with SourceKit, clients may get a SourceFileSyntax
in a serialized form from SourceKit response.  We need an API in the
SwiftSyntax framework to decode this form. This'll be a more common
pattern to get a Swift side syntax tree than invoking swiftc internally.

tools/SwiftSyntax/SwiftSyntax.swift

@@ -42,11 +42,27 @@ extension Syntax {
     guard result.wasSuccessful else {
       throw ParserError.swiftcFailed(result.exitCode, result.stderr)
     }
+    return try decodeSourceFileSyntax(result.stdoutData)
+  }
+
+  /// Decode a serialized form of SourceFileSyntax to a syntax node.
+  /// - Parameter content: The data of the serialized SourceFileSyntax.
+  /// - Returns: A top-level Syntax node representing the contents of the tree,
+  ///            if the parse was successful.
+  fileprivate static func decodeSourceFileSyntax(_ content: Data) throws -> SourceFileSyntax {
     let decoder = JSONDecoder()
-    let raw = try decoder.decode(RawSyntax.self, from: result.stdoutData)
+    let raw = try decoder.decode(RawSyntax.self, from: content)
     guard let file = makeSyntax(raw) as? SourceFileSyntax else {
       throw ParserError.invalidFile
     }
     return file
   }
+
+  /// Decode a serialized form of SourceFileSyntax to a syntax node.
+  /// - Parameter content: The string content of the serialized SourceFileSyntax.
+  /// - Returns: A top-level Syntax node representing the contents of the tree,
+  ///            if the parse was successful.
+  public static func decodeSourceFileSyntax(_ content: String) throws -> SourceFileSyntax {
+    return try decodeSourceFileSyntax(content.data(using: .utf8)!)
+  }
 }