This change translates constants and constructors (minus type
casting).  With this change, shaders such as

    gl_Position = vec4(aposition, 0, 1);

are translated correctly.

Bug: angleproject:4889
Change-Id: I4463717cf880c6d05db179b98691d5cabc1a2d7c
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2920192
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Tim Van Patten <timvp@google.com>