[SPARK-23986][SQL] freshName can generate non-unique names

## What changes were proposed in this pull request?

We are using `CodegenContext.freshName` to get a unique name for any new variable we are adding. Unfortunately, this method currently fails to create a unique name when we request more than one instance of variables with starting name `name1` and an instance with starting name `name11`.

The PR changes the way a new name is generated by `CodegenContext.freshName` so that we generate unique names in this scenario too.

## How was this patch tested?

added UT

Author: Marco Gaido <[email protected]>

Closes apache#21080 from mgaido91/SPARK-23986.

Author: mgaido91

sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/codegen/CodeGenerator.scala

@@ -572,14 +572,9 @@ class CodegenContext {
     } else {
       s"${freshNamePrefix}_$name"
     }
-    if (freshNameIds.contains(fullName)) {
-      val id = freshNameIds(fullName)
-      freshNameIds(fullName) = id + 1
-      s"$fullName$id"
-    } else {
-      freshNameIds += fullName -> 1
-      fullName
-    }
+    val id = freshNameIds.getOrElse(fullName, 0)
+    freshNameIds(fullName) = id + 1
+    s"${fullName}_$id"
   }
 
   /**

sql/catalyst/src/test/scala/org/apache/spark/sql/catalyst/expressions/CodeGenerationSuite.scala

@@ -489,4 +489,14 @@ class CodeGenerationSuite extends SparkFunSuite with ExpressionEvalHelper {
       assert(!ctx.subExprEliminationExprs.contains(ref))
     }
   }
+
+  test("SPARK-23986: freshName can generate duplicated names") {
+    val ctx = new CodegenContext
+    val names1 = ctx.freshName("myName1") :: ctx.freshName("myName1") ::
+      ctx.freshName("myName11") :: Nil
+    assert(names1.distinct.length == 3)
+    val names2 = ctx.freshName("a") :: ctx.freshName("a") ::
+      ctx.freshName("a_1") :: ctx.freshName("a_0") :: Nil
+    assert(names2.distinct.length == 4)
+  }
 }