Impeller targets Vulkan 1.1 core, which has required extensions for 16 bit uniform buffer access.

I started pulling on this thread in flutter/engine#42818, but there are several subproblems that we haven't solved yet with 16 bit buffers on the CPU side.

• The underlying ABI that clang uses for _Float16 is missing for Android and Windows builds.
• Some targets are segfaulting during Vulkan initialization.
• MoltenVK is failing to load some Vulkan 1.1 core extensions, like VK_KHR_storage_buffer_storage_class. But support was added in 2018. Need to dig into why this isn't working on CI.