[NFC] stdlib: Separate type decl from members.

Author: nate-chandler

stdlib/public/core/CommandLine.swift

@@ -20,7 +20,9 @@ internal func _swift_stdlib_getUnsafeArgvArgc(_: UnsafeMutablePointer<Int32>)
 
 /// Command-line arguments for the current process.
 @frozen // namespace
-public enum CommandLine {
+public enum CommandLine {}
+
+extension CommandLine {
   /// The backing static variable for argument count may come either from the
   /// entry point or it may need to be computed e.g. if we're in the REPL.
   @usableFromInline