Remove raw value from SourcePresence

Author: ahoppen

Sources/SwiftSyntax/SourcePresence.swift

@@ -14,10 +14,10 @@
 ///
 /// A `missing` node does not mean, necessarily, that the source item is
 /// considered "implicit", but rather that it was not found in the source.
-public enum SourcePresence: String {
+public enum SourcePresence {
   /// The syntax was authored by a human and found, or was generated.
-  case present = "Present"
+  case present
 
   /// The syntax was expected or optional, but not found in the source.
-  case missing = "Missing"
+  case missing
 }