1. add '-Wfloat-conversion' in compiler flag for clang
2. fix existed implicit float conversion

Bug: angleproject:3728
Change-Id: I0dc07eeb74c5d6dc480c6f0aa88bc75ab98e4292
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1713741
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>