[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.

1 files changed, 1 insertions, 1 deletions

diff --git a/sql/catalyst/src/main/antlr4/org/apache/spark/sql/catalyst/parser/SqlBase.g4 b/sql/catalyst/src/main/antlr4/org/apache/spark/sql/catalyst/parser/SqlBase.g4
index fcca11c69f..bd05855f0a 100644
--- a/sql/catalyst/src/main/antlr4/org/apache/spark/sql/catalyst/parser/SqlBase.g4
+++ b/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