[SQL] Add String option for DSL AS

Author: Michael Armbrust <michael@databricks.com>

Closes #3097 from marmbrus/asString and squashes the following commits:

6430520 [Michael Armbrust] Add String option for DSL AS

sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/dsl/package.scala

@@ -110,7 +110,8 @@ package object dsl {
     def asc = SortOrder(expr, Ascending)
     def desc = SortOrder(expr, Descending)
 
-    def as(s: Symbol) = Alias(expr, s.name)()
+    def as(alias: String) = Alias(expr, alias)()
+    def as(alias: Symbol) = Alias(expr, alias.name)()
   }
 
   trait ExpressionConversions {