GLSL basic, vector and matrix constructors can convert between types.
This was already done for constants used in constructors.  This change
implements the cast for non-constant expressions.

Bug: angleproject:4889
Change-Id: I0a8c1a6e97ffced0d1652032a41fb87c70be16ca
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2999022Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>