build-llvm: install clang-scan-dep tool for c++ modules

Compilers and Buildsystems have early support for c++ modules,
see [1],[2].
`clang-scan-dep` is necessary for Integration in CMake, potentially
other buildsystems in the future.

Using standard modules like from libc++ is still not solved thoroughly,
and module support from libc++ is marked experimental, documentation
how to use them is WIP [3].

[1]: https://www.kitware.com/import-cmake-the-experiment-is-over
[2]: https://cmake.org/cmake/help/latest/manual/cmake-cxxmodules.7.html
[3]: llvm/llvm-project#80601

Author: nolange

build-llvm.sh

@@ -322,5 +322,6 @@ cmake \
 
 cmake --build . ${CORES:+-j${CORES}}
 cmake --install . --strip
+cmake --install . --strip --component clang-scan-deps
 
 cp ../LICENSE.TXT $PREFIX