[Debug] Add _DebugDescription macro (#69626)

Implementation of the DebugDescription macro pitched on the forums: 
https://forums.swift.org/t/pitch-debug-description-macro/67711. In this initial commit, 
the macro is named `_DebugDescription` to indicate it's internal use at this time, 
pending Swift Evolution.

rdar://115180949

Author: kastiglione

lib/Macros/Sources/SwiftMacros/CMakeLists.txt

@@ -12,6 +12,7 @@
 
 add_swift_macro_library(SwiftMacros
   OptionSetMacro.swift
+  DebugDescriptionMacro.swift
   SWIFT_DEPENDENCIES
     SwiftDiagnostics
     SwiftSyntax