Change proxy toolchain to clang

Given

• clang is a first class citizen in upstream envoy and istio community
• some tests are clang only, and we essentially rely on upstream to run them
• clang was not used when we started OSSM because it wasn't available in brew/rhel at the time

It's worth to try switching to clang so that we are better align with those upstream communities, and we could eventually benefit from upstream improvements that are clang-specific.