[Serialization] When crashing, note if mix-and-match may be to blame.

That is, a Swift 3 target imported into a Swift 4 context or vice
versa. This requires serializing the compatibility mode explicitly,
instead of including it in the textual version string that's only
for debugging.

include/swift/Serialization/ModuleFile.h

@@ -72,6 +72,9 @@ class ModuleFile : public LazyMemberLoader {
   /// The target the module was built for.
   StringRef TargetTriple;
 
+  /// The Swift compatibility version in use when this module was built.
+  StringRef CompatibilityVersion;
+
   /// The data blob containing all of the module's identifiers.
   StringRef IdentifierData;