[clang] [Driver,CodeGen] Support -mtls-dialect= (PR #79256)

[Fangrui Song via cfe-commits]

MaskRay wrote:

> What's the expected interaction here with LTO? Modifying TargetOptions has no effect if we're generating a bitcode file.

Thanks for reminding me this. There seems no motivation to mix trad/desc, so I think we can do it the `-ffunction-sections`/`-femulated-tls` way by passing an option to the linker...

https://github.com/llvm/llvm-project/pull/79256