[Constraint solver] Switch coercion to be solution-based.

Rather than spinning up a new constraint system when performing an "as"
coercion, perform the coercion with the known solution, because we
already did all of the work to figure out how to perform the coercion.

Author: DougGregor

lib/Sema/CSApply.cpp

@@ -3530,12 +3530,9 @@ namespace {
         // Convert the subexpression.
         Expr *sub = expr->getSubExpr();
 
-        solution.setExprTypes(sub);
-
-        if (TypeChecker::convertToType(sub, toType, cs.DC))
+        sub = solution.coerceToType(sub, toType, cs.getConstraintLocator(sub));
+        if (!sub)
           return nullptr;
-          
-        cs.cacheExprTypes(sub);
 
         expr->setSubExpr(sub);
         cs.setType(expr, toType);