Add symbol table function to get TFunction for a built-in op 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>