[AST] Make it possible to find what Swift version was used to build a module

For imported modules the version is empty because they don't carry
this information.

include/swift/AST/Module.h

@@ -1147,6 +1147,10 @@ public:
 
   SourceRange getSourceRange() const { return SourceRange(); }
 
+  /// Returns the language version that was used to compile this module.
+  /// An empty `Version` is returned if the information is not available.
+  version::Version getLanguageVersionBuiltWith() const;
+
   static bool classof(const DeclContext *DC) {
     if (auto D = DC->getAsDecl())
       return classof(D);