[clang] [llvm] [LoongArch] Support LoongArch-specific amswap[_db].{b/h} and amadd[_db].{b/h} instructions (PR #113255)

[Chris Apple via cfe-commits]

cjappl wrote:

Just a heads up, I think this broke documentation builds. After bisecting it points to this commit.

```
ninja -C build docs-clang-html
FAILED: tools/clang/docs/CMakeFiles/docs-clang-html ~/code/radsan_cjappl/llvm-project/build/tools/clang/docs/CMakeFiles/docs-clang-html
cd ~/code/radsan_cjappl/llvm-project/build/tools/clang/docs && /opt/homebrew/Cellar/cmake/3.28.3/bin/cmake -E env ~/.virtualenvs/llvm/bin/sphinx-build -b htm
l -d ~/code/radsan_cjappl/llvm-project/build/tools/clang/docs/_doctrees-clang-html -q -t builder-html -D version=20 -D release=20.0.0git -tPreRelease -W ~co
de/radsan_cjappl/llvm-project/build/tools/clang/docs ~code/radsan_cjappl/llvm-project/build/tools/clang/docs/html

Warning, treated as error:
~/code/radsan_cjappl/llvm-project/build/tools/clang/docs/ClangCommandLineReference.rst:4819:Unknown target name: "amswap". [docutils]
```

See an example here:
https://github.com/llvm/llvm-project/actions/runs/11502212789/job/32016658426?pr=112727

https://github.com/llvm/llvm-project/pull/113255