[clang] [Clang][Xtensa] Add Xtensa target. (PR #118008)

[Eli Friedman via cfe-commits]

efriedma-quic wrote:

For reference, the RFC for the LLVM backend was https://discourse.llvm.org/t/rfc-request-for-upstream-tensilica-xtensa-esp32-backend/65355 .  Given there's an LLVM backend, I don't think we need a separate clang RFC?  Assuming the target doesn't require any exotic frontend features, we'd inevitably reach the same conclusion as the LLVM backend RFC.

That said, I am a little concerned that progress on the upstreaming is moving excessively slowly; it's been over two years since the original RFC.  New backends don't normally take that long... what exactly has been happening?

https://github.com/llvm/llvm-project/pull/118008