ModuleInterface: refactor out a utility to extract compiler flags from a module interface file. NFC

include/swift/Serialization/SerializedModuleLoader.h

@@ -461,7 +461,9 @@ public:
   }
 };
 
-
+/// Extract compiler arguments from an interface file buffer.
+bool extractCompilerFlagsFromInterface(StringRef buffer, llvm::StringSaver &ArgSaver,
+                                       SmallVectorImpl<const char *> &SubArgs);
 } // end namespace swift
 
 #endif