[CUDA] Added conversion functions to builtin vars.

This is needed to compile some headers in CUDA-11 that assume that threadIdx is
implicitly convertible to dim3. With NVCC, threadIdx is uint3 and there's
dim3(uint3) constructor. Clang uses a special type for the builtin variables, so
that path does not work. Instead, this patch adds conversion function to the
builtin variable classes. that will allow them to be converted to dim3 and uint3.

Differential Revision: https://reviews.llvm.org/D88250