Rework effects checking to maintain thrown type information.

While here, eliminate some redundancy in the implementation of
effects checking be consolidating the logic for computing a
`Classification` from a particular expression or other entity.

Author: DougGregor

lib/Sema/CSApply.cpp

@@ -8892,7 +8892,7 @@ static Expr *wrapAsyncLetInitializer(
       ConstraintSystem &cs, Expr *initializer, DeclContext *dc) {
   // Form the autoclosure type. It is always 'async', and will be 'throws'.
   Type initializerType = initializer->getType();
-  bool throws = TypeChecker::canThrow(initializer);
+  bool throws = TypeChecker::canThrow(cs.getASTContext(), initializer);
   auto extInfo = ASTExtInfoBuilder()
     .withAsync()
     .withConcurrent()