[Debug] Conditional _DebugDescription macro declaration (#70351)

Make `_DebugDescription` macro conditional upon macro availability in the build.

The `Macros` feature is based on the value of `SWIFT_BUILD_SWIFT_SYNTAX`.

See #69626 (comment)

Author: kastiglione

stdlib/public/core/DebuggerSupport.swift

@@ -12,6 +12,9 @@
 
 import SwiftShims
 
+// Macros are disabled when Swift is built without swift-syntax.
+#if $Macros && hasAttribute(attached)
+
 /// Converts description definitions to a debugger type summary.
 ///
 /// This macro converts compatible `debugDescription` (or `description`)
@@ -42,6 +45,8 @@ public macro _DebugDescription() =
 public macro _DebugDescriptionProperty(_ debugIdentifier: String, _ computedProperties: [String]) =
   #externalMacro(module: "SwiftMacros", type: "_DebugDescriptionPropertyMacro")
 
+#endif
+
 #if SWIFT_ENABLE_REFLECTION
 
 @frozen // namespace