Documentation: add minimum clang/llvm version

Based on a vote at the LLVM BoF at Plumbers 2020, we decided to start
small, supporting just one formal upstream release of LLVM for now.

We can probably widen the support window of supported versions over
time.  Also, note that LLVM's release process is different than GCC's.
GCC tends to have 1 major release per year while releasing minor updates
to the past 3 major versions.  LLVM tends to support one major release
and one minor release every six months.
Signed-off-by: NNick Desaulniers <ndesaulniers@google.com>
Tested-by: NGustavo A. R. Silva <gustavoars@kernel.org>
Tested-by: NNathan Chancellor <natechancellor@gmail.com>
Reviewed-by: NKees Cook <keescook@chromium.org>
Reviewed-by: NNathan Chancellor <natechancellor@gmail.com>
Reviewed-by: NMasahiro Yamada <masahiroy@kernel.org>
Acked-by: NWill Deacon <will@kernel.org>
Link: https://lore.kernel.org/r/20200826191555.3350406-1-ndesaulniers@google.comSigned-off-by: NJonathan Corbet <corbet@lwn.net>

Documentation/kbuild/llvm.rst

+.. _kbuild_llvm:
+
 ==============================
 Building Linux with Clang/LLVM
 ==============================
@@ -73,6 +75,8 @@ Getting Help
 - `Wiki <https://github.com/ClangBuiltLinux/linux/wiki>`_
 - `Beginner Bugs <https://github.com/ClangBuiltLinux/linux/issues?q=is%3Aopen+is%3Aissue+label%3A%22good+first+issue%22>`_
 
+.. _getting_llvm:
+
 Getting LLVM
 -------------
 

Documentation/process/changes.rst

@@ -30,6 +30,7 @@ you probably needn't concern yourself with pcmciautils.
         Program        Minimal version       Command to check the version
 ====================== ===============  ========================================
 GNU C                  4.9              gcc --version
+Clang/LLVM (optional)  10.0.1           clang --version
 GNU make               3.81             make --version
 binutils               2.23             ld -v
 flex                   2.5.35           flex --version
@@ -68,6 +69,15 @@ GCC
 The gcc version requirements may vary depending on the type of CPU in your
 computer.
 
+Clang/LLVM (optional)
+---------------------
+
+The latest formal release of clang and LLVM utils (according to
+`releases.llvm.org <https://releases.llvm.org>`_) are supported for building
+kernels. Older releases aren't guaranteed to work, and we may drop workarounds
+from the kernel that were used to support older versions. Please see additional
+docs on :ref:`Building Linux with Clang/LLVM <kbuild_llvm>`.
+
 Make
 ----
 
@@ -331,6 +341,11 @@ gcc
 
 - <ftp://ftp.gnu.org/gnu/gcc/>
 
+Clang/LLVM
+----------
+
+- :ref:`Getting LLVM <getting_llvm>`.
+
 Make
 ----