The glslang public includes dir contains headers with implicit
conversion issues. Add -Wno-conversion to glslang's public config.

Bug: 60140
Bug: 58162
Change-Id: Iec27cb4242e9fdceddd6a3e02044a0bccfa0ce36
Reviewed-on: https://fuchsia-review.googlesource.com/c/third_party/glslang/+/429054Reviewed-by: Petr Hosek <phosek@google.com>
Co-authored-by: Shai Barack <shayba@google.com>