[SPARK-18465] Add 'IF EXISTS' clause to 'UNCACHE' to not throw exceptions when table doesn't exist

## What changes were proposed in this pull request?

While this behavior is debatable, consider the following use case:
```sql
UNCACHE TABLE foo;
CACHE TABLE foo AS
SELECT * FROM bar
```
The command above fails the first time you run it. But I want to run the command above over and over again, and I don't want to change my code just for the first run of it.
The issue is that subsequent `CACHE TABLE` commands do not overwrite the existing table.

Now we can do:
```sql
UNCACHE TABLE IF EXISTS foo;
CACHE TABLE foo AS
SELECT * FROM bar
```

## How was this patch tested?

Unit tests

Author: Burak Yavuz <brkyvz@gmail.com>

Closes #15896 from brkyvz/uncache.

sql/catalyst/src/main/antlr4/org/apache/spark/sql/catalyst/parser/SqlBase.g4

@@ -142,7 +142,7 @@ statement
     | REFRESH TABLE tableIdentifier                                    #refreshTable
     | REFRESH .*?                                                      #refreshResource
     | CACHE LAZY? TABLE tableIdentifier (AS? query)?                   #cacheTable
-    | UNCACHE TABLE tableIdentifier                                    #uncacheTable
+    | UNCACHE TABLE (IF EXISTS)? tableIdentifier                       #uncacheTable
     | CLEAR CACHE                                                      #clearCache
     | LOAD DATA LOCAL? INPATH path=STRING OVERWRITE? INTO TABLE
         tableIdentifier partitionSpec?                                 #loadData

sql/core/src/main/scala/org/apache/spark/sql/execution/SparkSqlParser.scala

@@ -233,7 +233,7 @@ class SparkSqlAstBuilder(conf: SQLConf) extends AstBuilder {
    * Create an [[UncacheTableCommand]] logical plan.
    */
   override def visitUncacheTable(ctx: UncacheTableContext): LogicalPlan = withOrigin(ctx) {
-    UncacheTableCommand(visitTableIdentifier(ctx.tableIdentifier))
+    UncacheTableCommand(visitTableIdentifier(ctx.tableIdentifier), ctx.EXISTS != null)
   }
 
   /**

sql/core/src/main/scala/org/apache/spark/sql/execution/command/cache.scala

@@ -19,6 +19,7 @@ package org.apache.spark.sql.execution.command
 
 import org.apache.spark.sql.{Dataset, Row, SparkSession}
 import org.apache.spark.sql.catalyst.TableIdentifier
+import org.apache.spark.sql.catalyst.analysis.NoSuchTableException
 import org.apache.spark.sql.catalyst.plans.QueryPlan
 import org.apache.spark.sql.catalyst.plans.logical.LogicalPlan
 
@@ -49,10 +50,17 @@ case class CacheTableCommand(
 }
 
 
-case class UncacheTableCommand(tableIdent: TableIdentifier) extends RunnableCommand {
+case class UncacheTableCommand(
+    tableIdent: TableIdentifier,
+    ifExists: Boolean) extends RunnableCommand {
 
   override def run(sparkSession: SparkSession): Seq[Row] = {
-    sparkSession.catalog.uncacheTable(tableIdent.quotedString)
+    val tableId = tableIdent.quotedString
+    try {
+      sparkSession.catalog.uncacheTable(tableId)
+    } catch {
+      case _: NoSuchTableException if ifExists => // don't throw
+    }
     Seq.empty[Row]
   }
 }

sql/hive/src/test/scala/org/apache/spark/sql/hive/CachedTableSuite.scala

@@ -101,13 +101,16 @@ class CachedTableSuite extends QueryTest with SQLTestUtils with TestHiveSingleto
     sql("DROP TABLE IF EXISTS nonexistantTable")
   }
 
-  test("correct error on uncache of nonexistant tables") {
+  test("uncache of nonexistant tables") {
+    // make sure table doesn't exist
+    intercept[NoSuchTableException](spark.table("nonexistantTable"))
     intercept[NoSuchTableException] {
       spark.catalog.uncacheTable("nonexistantTable")
     }
     intercept[NoSuchTableException] {
       sql("UNCACHE TABLE nonexistantTable")
     }
+    sql("UNCACHE TABLE IF EXISTS nonexistantTable")
   }
 
   test("no error on uncache of non-cached table") {