[clang] [clang][driver] Add -mtls-dialect option (PR #79031)

[Paul Kirth via cfe-commits]

ilovepi wrote:

Note: codegen changes to support meaningful `-mtls-dialect` changes are introduced in https://github.com/llvm/llvm-project/pull/66915. 

https://github.com/llvm/llvm-project/pull/79031