[SYCL] Add vec<bfloat16> support to math builtins (intel#14002)

This PR
1. Updates extension to add math builtins for `vec<bfloat16>` and
corresponding swizzles.
2. Implements **unoptimized** support for `vec<bloat16>`/swizzles in
math builtins and adds a test case for the same.
3. Adds a test to check the device code generated for `vec<bloat16>`
math builtins.

I will make a follow-up PR to optimize `vec<bfloat16>` math builtins. I
think we can use elementwise builtins for `ext_vector_type`
(https://clang.llvm.org/docs/LanguageExtensions.html#vectors-and-extended-vectors)
to optimize `vec<bfloat16>` math builtins. The device code test case
will help visualizing/reviewing math builtin optimizations.

Author: uditagarwal97