[docs] Add llvm & clang release notes for LoongArch

[SixWeining]

I'm not sure if there is any format issue. Is there any way to verify? e.g. how to generate .html like https://llvm.org/docs/ReleaseNotes.html?

[SixWeining]

In D141750#4053469, @SixWeining wrote:

I'm not sure if there is any format issue. Is there any way to verify? e.g. how to generate .html like https://llvm.org/docs/ReleaseNotes.html?

Now I find it: llvm/docs/README.txt.

[MaskRay]

pip3 install --user recommonmark sphinx # sphinx-automodapi for lldb

cmake ...

ninja ... docs-llvm-html

[SixWeining]

Fix the issue "Title underline too short" reported by sphinx-build.

[SixWeining]

In D141750#4053473, @MaskRay wrote:

pip3 install --user recommonmark sphinx # sphinx-automodapi for lldb

cmake ...

ninja ... docs-llvm-html

Thanks.

[rengolin]

Nice! Thanks for the detailed reporting. LGTM.

clang/docs/ReleaseNotes.rst

 ...
 
 CUDA Support in Clang
 ---------------------
 
 - Clang now supports CUDA SDK up to 11.8
 - Added support for targeting sm_{87,89,90} GPUs.
 
+LoongArch Support in Clang
+--------------------------
+- Clang now supports LoongArch. Along with the backend, clang is able to build a
+  large corpus of Linux applications. Test-suite 100% pass.
+- Support basic option ``-march=`` which is used to select the target
+  architecture, i.e. the basic set of ISA modules to be enabled. Possible values
+  are ``loongarch64`` and ``la464``.
+- Support basic option ``-mabi=`` which is used to select the base ABI type.
+  Possible values are ``lp64d``, ``lp64f``, ``lp64s``, ``ilp32d``, ``ilp32f``
+  and ``ilp32s``.
+- Support extended options: ``-msoft-float``, ``-msingle-float``, ``-mdouble-float`` and ``mfpu=``.
+  See `LoongArch toolchain conventions <https://loongson.github.io/LoongArch-Documentation/LoongArch-toolchain-conventions-EN.html>`_.
+
 RISC-V Support in Clang
 -----------------------
 - ``sifive-7-rv32`` and ``sifive-7-rv64`` are no longer supported for ``-mcpu``.
   Use ``sifive-e76``, ``sifive-s76``, or ``sifive-u74`` instead.
 - Native detections via ``-mcpu=native`` and ``-mtune=native`` are supported.
 - Fix interaction of ``-mcpu`` and ``-march``, RISC-V backend will take the
   architecture extension union of ``-mcpu`` and ``-march`` before, and now will
   take architecture extensions from ``-march`` if both are given.

llvm/docs/ReleaseNotes.rst

.. NOTE
    functionality, or simply have a lot to talk about), see the `NOTE` below
    for adding a new subsection.
 
 *  The ``readnone`` calls which are crossing suspend points in coroutines will
    not be merged. Since ``readnone`` calls may access thread id and thread id
    is not a constant in coroutines. This decision may cause unnecessary
    performance regressions and we plan to fix it in later versions.
 
+*  The LoongArch target is promoted to "official" (see below for more details).
+
 * ...
 
 Update on required toolchains to build LLVM
 -------------------------------------------
 
 LLVM is now built with C++17 by default. This means C++17 can be used in
 the code base.
 

 Changes to the DirectX Backend
 ------------------------------
 
 Changes to the Hexagon Backend
 ------------------------------
 
 * ...
 
+Changes to the LoongArch Backend
+--------------------------------
+
+* The LoongArch target is no longer "experimental"! It's now built by default,
+  rather than needing to be enabled with ``LLVM_EXPERIMENTAL_TARGETS_TO_BUILD``.
+
+* The backend has full codegen support for the base (both integer and
+  floating-point) instruction set and it conforms to psABI v2. Testing has been
+  performed with Linux, including native compilation of a large corpus of Linux
+  applications.
+
+* Support GHC calling convention.
+
 Changes to the MIPS Backend
 ---------------------------
 
 * ...
 
 Changes to the PowerPC Backend
 ------------------------------
 
* ``llvm-readobj --elf-output-style=JSON`` no longer prefixes each JSON object
   accordingly.
 
 * ``llvm-objdump`` now uses ``--print-imm-hex`` by default, which brings its
   default behavior closer in line with ``objdump``.
 
 Changes to LLDB
 ---------------------------------
 
+* Initial support for debugging Linux LoongArch 64-bit binaries.
+
 Changes to Sanitizers
 ---------------------
 
+* Many Sanitizers (asan, fuzzer, lsan, safestack, scudo, tsan, ubsan) have
+  support for Linux LoongArch 64-bit variant. Some of them may be rudimentary.
 
 Other Changes
 -------------
 
 External Open Source Projects Using LLVM 15
 ===========================================
 
 * A project...