Built-in function parameter qualifiers are stored in the symbol table.
Some AST traversers need the qualifier information for ops to
determine whether a node is being written to. Add an utility function
that maps a TIntermAggregate node to a symbol table entry, so that the
traversers can get to this information in a convenient way.

This will be necessary for adding more built-ins that have out
parameters from ESSL 3.10.

BUG=angleproject:1730
TEST=angle_unittests

Change-Id: I4bc622d70b2326a04cc858ff1258c22320c590dc
Reviewed-on: https://chromium-review.googlesource.com/431109
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>