[SPARK-16678][SPARK-16677][SQL] Fix two View-related bugs

## What changes were proposed in this pull request?
**Issue 1: Disallow Creating/Altering a View when the same-name Table Exists (without IF NOT EXISTS)**
When we create OR alter a view, we check whether the view already exists. In the current implementation, if a table with the same name exists, we treat it as a view. However, this is not the right behavior. We should follow what Hive does. For example,
```
hive> CREATE TABLE tab1 (id int);
OK
Time taken: 0.196 seconds
hive> CREATE OR REPLACE VIEW tab1 AS SELECT * FROM t1;
FAILED: SemanticException [Error 10218]: Existing table is not a view
 The following is an existing table, not a view: default.tab1
hive> ALTER VIEW tab1 AS SELECT * FROM t1;
FAILED: SemanticException [Error 10218]: Existing table is not a view
 The following is an existing table, not a view: default.tab1
hive> CREATE VIEW IF NOT EXISTS tab1 AS SELECT * FROM t1;
OK
Time taken: 0.678 seconds
```

**Issue 2: Strange Error when Issuing Load Table Against A View**
Users should not be allowed to issue LOAD DATA against a view. Currently, when users doing it, we got a very strange runtime error. For example,
```SQL
LOAD DATA LOCAL INPATH "$testData" INTO TABLE $viewName
```
```
java.lang.reflect.InvocationTargetException was thrown.
java.lang.reflect.InvocationTargetException
	at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)
	at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
	at java.lang.reflect.Method.invoke(Method.java:606)
	at org.apache.spark.sql.hive.client.Shim_v0_14.loadTable(HiveShim.scala:680)
```
## How was this patch tested?
Added test cases

Author: gatorsmile <gatorsmile@gmail.com>

Closes #14314 from gatorsmile/tableDDLAgainstView.

1 files changed, 3 insertions, 3 deletions

diff --git a/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/catalog/SessionCatalog.scala b/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/catalog/SessionCatalog.scala
index 134fc4e698..1856dc4d64 100644
--- a/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/catalog/SessionCatalog.scala
+++ b/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/catalog/SessionCatalog.scala
@@ -443,12 +443,12 @@ class SessionCatalog(
   }
 
   /**
-   * Return whether a table with the specified name exists.
+   * Return whether a table/view with the specified name exists.
    *
-   * Note: If a database is explicitly specified, then this will return whether the table
+   * Note: If a database is explicitly specified, then this will return whether the table/view
    * exists in that particular database instead. In that case, even if there is a temporary
    * table with the same name, we will return false if the specified database does not
-   * contain the table.
+   * contain the table/view.
    */
   def tableExists(name: TableIdentifier): Boolean = synchronized {
     val db = formatDatabaseName(name.database.getOrElse(currentDb))