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

[Alexey Gerenkov via cfe-commits]

gerekon wrote:

> I don't know what this target IS, did we have an RFC to approve this in Clang as a target? Do we have sufficient interest in the project to maintain it/keep it?

This is a part of our work for upstreaming Xtensa target support. https://github.com/espressif/llvm-project/issues/4
We finished with base functionality in the backend. And now we are preparing PRs with Xtensa architecture features.
This patch adds support for Xtensa target in Clang. It'd be strange to have backend support in LLVM and do noit have it in Clang.

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