$ echo "int main() { }" > a.c
$ nix shell nixpkgs#llvmPackages.clangUseLLVM -c clang /tmp/a.c
/nix/store/039g378vc3pc3dvi9dzdlrd0i4q93qwf-binutils-2.39/bin/ld: cannot find -l:libunwind.so: No such file or directory
clang-11: error: linker command failed with exit code 1 (use -v to see invocation)

$ NIX_DEBUG=1 nix shell nixpkgs#llvmPackages.clangUseLLVM -c clang a.c
HARDENING: disabled flags: pie pic format stackprotector fortify strictoverflow
extra flags before to /nix/store/39yzdj9a7l8aw317l8la4kp6aalczsq7-clang-11.1.0/bin/clang:
  -Wl\,-dynamic-linker=/nix/store/4nlgxhb09sdr51nc9hdm8az5b08vzkgx-glibc-2.35-163/lib/ld-linux-x86-64.so.2
  -target
  x86_64-unknown-linux-gnu
original flags to /nix/store/39yzdj9a7l8aw317l8la4kp6aalczsq7-clang-11.1.0/bin/clang:
  a.c
extra flags after to /nix/store/39yzdj9a7l8aw317l8la4kp6aalczsq7-clang-11.1.0/bin/clang:
  -B/nix/store/4nlgxhb09sdr51nc9hdm8az5b08vzkgx-glibc-2.35-163/lib/
  -idirafter
  /nix/store/4pqv2mwdn88h7xvsm7a5zplrd8sxzvw0-glibc-2.35-163-dev/include
  --gcc-toolchain=/nix/store/1gf2flfqnpqbr1b4p4qz2f72y42bs56r-gcc-11.3.0
  -B/nix/store/fbc6m1s0638pf3vk5wgmcsbap8f04nar-clang-11.1.0-lib/lib
  -rtlib=compiler-rt
  -Wno-unused-command-line-argument
  -B/nix/store/lkckixdqkvwscdr5jf0v07w77lhlm70f-compiler-rt-libc-11.1.0/lib
  --unwindlib=libunwind
  -resource-dir=/nix/store/6bmyzky9i7rd2dp3lxc47rlabrhcl46y-clang-wrapper-11.1.0/resource-root
  -B
  /nix/store/6bmyzky9i7rd2dp3lxc47rlabrhcl46y-clang-wrapper-11.1.0/resource-root/lib
  -B/nix/store/6bmyzky9i7rd2dp3lxc47rlabrhcl46y-clang-wrapper-11.1.0/bin/
  -L/nix/store/4nlgxhb09sdr51nc9hdm8az5b08vzkgx-glibc-2.35-163/lib
  -L/nix/store/fbc6m1s0638pf3vk5wgmcsbap8f04nar-clang-11.1.0-lib/lib
HARDENING: disabled flags: relro pie bindnow
extra flags before to /nix/store/039g378vc3pc3dvi9dzdlrd0i4q93qwf-binutils-2.39/bin/ld:
  ''
original flags to /nix/store/039g378vc3pc3dvi9dzdlrd0i4q93qwf-binutils-2.39/bin/ld:
  --eh-frame-hdr
  -m
  elf_x86_64
  -o
  a.out
  /nix/store/4nlgxhb09sdr51nc9hdm8az5b08vzkgx-glibc-2.35-163/lib/crt1.o
  /nix/store/4nlgxhb09sdr51nc9hdm8az5b08vzkgx-glibc-2.35-163/lib/crti.o
  /nix/store/6bmyzky9i7rd2dp3lxc47rlabrhcl46y-clang-wrapper-11.1.0/resource-root/lib/linux/clang_rt.crtbegin-x86_64.o
  -L/nix/store/4nlgxhb09sdr51nc9hdm8az5b08vzkgx-glibc-2.35-163/lib
  -L/nix/store/fbc6m1s0638pf3vk5wgmcsbap8f04nar-clang-11.1.0-lib/lib
  -L/nix/store/1gf2flfqnpqbr1b4p4qz2f72y42bs56r-gcc-11.3.0/lib64/gcc/x86_64-unknown-linux-gnu/11.3.0
  -L/nix/store/1gf2flfqnpqbr1b4p4qz2f72y42bs56r-gcc-11.3.0/lib64/gcc/x86_64-unknown-linux-gnu/11.3.0/../../../../lib64
  -L/nix/store/1gf2flfqnpqbr1b4p4qz2f72y42bs56r-gcc-11.3.0/lib64/gcc/x86_64-unknown-linux-gnu/11.3.0/../../..
  -dynamic-linker=/nix/store/4nlgxhb09sdr51nc9hdm8az5b08vzkgx-glibc-2.35-163/lib/ld-linux-x86-64.so.2
  /tmp/a-804f0d.o
  /nix/store/6bmyzky9i7rd2dp3lxc47rlabrhcl46y-clang-wrapper-11.1.0/resource-root/lib/linux/libclang_rt.builtins-x86_64.a
  --as-needed
  -l:libunwind.so
  --no-as-needed
  -lc
  /nix/store/6bmyzky9i7rd2dp3lxc47rlabrhcl46y-clang-wrapper-11.1.0/resource-root/lib/linux/libclang_rt.builtins-x86_64.a
  --as-needed
  -l:libunwind.so
  --no-as-needed
  /nix/store/6bmyzky9i7rd2dp3lxc47rlabrhcl46y-clang-wrapper-11.1.0/resource-root/lib/linux/clang_rt.crtend-x86_64.o
  /nix/store/4nlgxhb09sdr51nc9hdm8az5b08vzkgx-glibc-2.35-163/lib/crtn.o
extra flags after to /nix/store/039g378vc3pc3dvi9dzdlrd0i4q93qwf-binutils-2.39/bin/ld:
  -rpath
  /nix/store/4nlgxhb09sdr51nc9hdm8az5b08vzkgx-glibc-2.35-163/lib
/nix/store/039g378vc3pc3dvi9dzdlrd0i4q93qwf-binutils-2.39/bin/ld: cannot find -l:libunwind.so: No such file or directory
clang-11: error: linker command failed with exit code 1 (use -v to see invocation)