clang-tools-extra has been moved to a separate "extra" output due to the
size of the statically linked binaries. Unfortunately, this prevents tools
from finding the header files stored in the default clang "out" output:

--8<---------------cut here---------------start------------->8---
$ guix describe
Generation 1 Feb 08 2022 14:30:38 (current)
  guix 59892f1
    repository URL: https://git.savannah.gnu.org/git/guix.git
    branch: master
    commit: 59892f105585a98bcaaa4abc99142944cb171acf

$ cat main.cpp
#include <iostream>

int main() {
    std::cout << "Hello, World!" << std::endl;
    return 0;
}

$ guix shell clang clang:extra

$ clang-tidy main.cpp --
1776 warnings and 1 error generated.
Error while processing /efs/devel/hello/src/main.cpp.
/gnu/store/i86pbiqnnz0c2313616jdbsbdgp7qvav-profile/include/wchar.h:35:10:
error: 'stddef.h' file not found [clang-diagnostic-error]
#include <stddef.h>
         ^~~~~~~~~~
Suppressed 1776 warnings (1776 in non-user code).
Use -header-filter=.* to display errors from all non-system headers. Use
-system-headers to display errors from system headers as well.
Found compiler error(s).
--8<---------------cut here---------------end--------------->8---

After applying the attached patch the tool does find the necessary header
file (at /gnu/store/*-clang-13.0.1/lib/clang/13.0.1/include/stddef.h):

--8<---------------cut here---------------start------------->8---
$ ./pre-inst-env guix shell clang

$ clang-tidy main.cpp --
1971 warnings generated.
Suppressed 1971 warnings (1971 in non-user code).
Use -header-filter=.* to display errors from all non-system headers. Use
-system-headers to display errors from system headers as well.
--8<---------------cut here---------------end--------------->8---

The patch builds clang with dynamic linkage and removes the separate
output. The new size of the full complement of dynamically linked libraries
and binaries is one-third the size of the original clang with extras
excluded.

--8<---------------cut here---------------start------------->8---
$ guix size clang
store item                                                       total
 self
/gnu/store/ylbcqp3yjha97g450g33cz9grsg0ic2w-clang-13.0.1          1247.5
628.0  50.3%
...
total: 1247.5 MiB

$ ./pre-inst-env guix size clang
store item                                                       total
 self
/gnu/store/fxf9gyx7c790kbbzhyyamdf27g8yjb9v-clang-13.0.1           842.1
222.6  26.4%
...
total: 842.1 MiB
--8<---------------cut here---------------end--------------->8---

I successfully built all dependent packages of the affected clang versions
(clang@10 through 13) except c-vise-2.4.0, for which I have included a
patch, and the following packages currently failing on master:
  clang-toolchain-8.0.0
  dub@1.7.2
  gtkd@3.9.0
  icedove-wayland@91.5
  itk-snap@3.8.0
  java-pep-adapter-2.1.23
  rdmd@2.077.1
  rust-aom-sys-0.3.0
  rust-bindgen-0.52.0
  rust-bindgen-0.55.1
  rust-clang-sys-0.26.4
  rust-sequoia-openpgp-0.9.0
  scregseg-0.1.1

Greg