Whether a function call is user-defined is not orthogonal to TOperator
associated with the call node - other ops than function calls can't be
user-defined. Because of this it makes sense to store the user-
definedness by having different TOperator enums for different types of
calls.

This patch also tags internal helper functions that have a raw
definition outside the AST with a separate TOperator enum. This way
they can be handled with logic that is easy to understand. Before this,
function calls like this left the user-defined bit unset, despite not
really being built-ins either. The EmulatePrecision traverser uses
this. This is also something that could be used to clean up built-in
emulation in the future.

BUG=angleproject:1490
TEST=angle_unittests

Change-Id: I597fcd9789d0cc22b689ef3ce5a0cc3f621d4859
Reviewed-on: https://chromium-review.googlesource.com/433443Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>