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

[Fangrui Song via cfe-commits]

MaskRay wrote:

Since TLSDialect only has 2 values and it's unlikely a future change will add new, I think sticking with a boolean codegenopt suffices.
For x86, we can probably support "gnu" as "gnu2" (TLSDESC) is a desired feature. (I added lld support, but I did not know codegen well enough to add LLVM part...)

I think #79256 is a more complete patch, with these ideas taken into account..
It may be a good candidate for the `release/18.x` branch.

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