This change implements function calls.  As a result, transform feedback
tests are enabled as well as support for transform feedback emulation
(which contains functions and if blocks).

`const` and opaque uniform function arguments take intermediate values,
while the rest take memory objects.  If the argument being passed is an
unindexed lvalue, it can be directly given to the function.  Otherwise a
temporary variable is made which is initialized by the parameter if
necessary (in and inout parameters) and later overwrites the parameter
if necessary (out and inout parameters).

Bug: angleproject:4889
Change-Id: I8976cdd17870c35d5a2daeed3c38de57ada931d9
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2930363
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Tim Van Patten <timvp@google.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>