[clang] [HLSL][SPIR-V] Add `-fspv-enable-maximal-reconvergence` flag to clang dxc. (PR #163474)

[Steven Perron via cfe-commits]

https://github.com/s-perron commented:

Generally LGTM. The only issue is the wording for documentation. You could also keep it simple as we did in DXC:

> Enables the MaximallyReconvergesKHR execution mode for this module.

https://github.com/llvm/llvm-project/pull/163474