Allow user set CUDA Clang

[jeremylt]

Purpose:

This PR lets the user specify the Clang CXX for CUDA JiT. If this is not valid or not set, then we attempt to detect the Clang version in the Rust toolchain. Finally we default to clang++ if neither is successful.

Closes: #1965

LLM/GenAI Disclosure:

None

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[jeremylt]

Ok, all set

To investigate in follow-up:

warning: feature `asm_experimental_arch` is declared but not used
 --> src/lib.rs:3:12
  |
3 | #![feature(asm_experimental_arch, abi_ptx, core_intrinsics)]
  |            ^^^^^^^^^^^^^^^^^^^^^
  |
  = note: `#[warn(unused_features)]` (part of `#[warn(unused)]`) on by default
warning: feature `abi_ptx` is declared but not used
 --> src/lib.rs:3:35
  |
3 | #![feature(asm_experimental_arch, abi_ptx, core_intrinsics)]
  |                                   ^^^^^^^
warning: `ex1-volume-rs` (lib) generated 2 warnings