Reduce clang size

The current size of clang binaries is quite large:

--- /nix/store/2cfpyiqgqsc4kns81r7zqkc0swp6jnmb-clang-ompss2-2023.05/bin -------------------------
                                              /..
  171.3 MiB [ 15.0% ########################]  clang-17
  139.7 MiB [ 12.2% ###################     ]  clang-scan-deps
  129.2 MiB [ 11.3% ##################      ]  clang-check
  114.6 MiB [ 10.0% ################        ]  lld
  114.4 MiB [ 10.0% ################        ]  clang-repl
  100.0 MiB [  8.7% ##############          ]  clang-linker-wrapper
   79.2 MiB [  6.9% ###########             ]  llvm-dwp
   38.0 MiB [  3.3% #####                   ]  clang-refactor
   36.7 MiB [  3.2% #####                   ]  clang-rename
   35.6 MiB [  3.1% ####                    ]  clang-extdef-mapping
   30.5 MiB [  2.7% ####                    ]  llvm-ml
   29.7 MiB [  2.6% ####                    ]  llvm-nm
   29.6 MiB [  2.6% ####                    ]  llvm-ar
   27.4 MiB [  2.4% ###                     ]  llvm-objdump
    8.1 MiB [  0.7% #                       ]  llvm-readobj
    7.7 MiB [  0.7% #                       ]  diagtool
    7.2 MiB [  0.6% #                       ]  llvm-pdbutil

Leaving 1.6 GB installation for each clang. Maybe we can trim the size down a bit by using BUILD_SHARED_LIBS or LLVM_BUILD_LLVM_DYLIB, as commented by @rpenacob . We may also want to enable ccache to accelerate the builds.