Fixed bug#11749345 (formerly bug#38813) - increasing memory consumption

when selecting from I_S and views exist, in SP.

Symptoms: re-execution of prepared statement (or statement in a stored
routine) which read from one of I_S tables and which in order to fill
this I_S table had to open a view led to increasing memory consumption.

What happened in this situation was that during the process of view
opening for purpose of I_S filling view-related structures (like its
LEX) were allocated on persistent MEM_ROOT of prepared statement (or 
stored routine). Since this MEM_ROOT is not freed until prepared
statement deallocation (or expulsion of stored routine from the cache)
and code responsible for filling I_S is not able to re-use results of
view opening from previous executions this allocation ended up in
memory hogging.

This patch solves the problem by ensuring that when a view opened
for the purpose of I_S filling all its structures are allocated on
non-persistent runtime MEM_ROOT. This is achieved by activating a
temporary Query_arena bound to this MEM_ROOT.
Since this step makes impossible linking of view structures into
LEX of our prepared statement (or stored routine statement) this
patch also changes code filling I_S table to install a proxy LEX
before trying to open a view or a table. Consequently some code
which was responsible for backing-up/restoring parts of LEX when
view/table was opened during filling of I_S table became redundant
and was removed.

This patch doesn't contain test case for this bug as it is hard
to test memory hogging in our test suite.