Fix leak of QString in V4 IR

Fix leak of QString in V4 IR

The destructors of the IR nodes won't be called because they come out of
a memory pool. Therefore we must store strings by pointer from the function's
string pool.

Change-Id: I841e801b81c871e8d08cf63ee1e053744c0bf4dc
Reviewed-by: Lars Knoll <lars.knoll@digia.com>