TABLE COMMENTS "... IS NOT BASE TABLE"

PROBLEM:
========
With DERIVED_MERGE disabled using the OPTIMIZER_SWITCH, the
INFORMATION_SCHEMA(I_S).TABLES.TABLE_COMMENT column shows a
strange comment stating that the view is not a BASE TABLE.

ANALYSIS:
=========
With DERIVED_MERGE disabled, the optimizer ends-up invoking
functions which request values for dynamic columns like
TABLE_ROWS, AVG_ROW_LENGTH, DATA_LENGTH, MAX_DATA_LENGTH,
INDEX_LENGTH, DATA_FREE, AUTO_INCREMENT, UPDATE_TIME,
CHECK_TIME and CHECKSUM from storage engine. This causes I_S
module to try and open the view treating it as a base table,
which is incorrect.

FIX:
====
Modifies the INFORMATION_SCHEMA.TABLES dynamic column
definitions to return NULL, if it finds a view.

Note:
=====
BUG#26203731 introduced time based caching by introducing a
new session variable "information_schema_stats_expiry". This
patch made INFORMATION_SCHEMA.TABLES dynamic columns to
return 0 instead of NULL.

Change-Id: Idaf62684f0dbfa065aa89486ef1be57e07ab46c2