clang: install clang-scan-deps

clang-scan-deps is used to generate dependency information from C++20
modules according to proposed standard ISO/IEC WG21 P1689R5[0]. It is
required by common build tools (e.g., CMake) to build C++ sources
that use modules.

Since this is a core build tool, install it by default, not gated
behind MK_CLANG_EXTRAS.

[0] https://www.open-std.org/JTC1/SC22/WG21/docs/papers/2022/p1689r5.html

MFC after: 3 days
Reviewed by: kevans, dim
Approved by: kevans (mentor)
Requested by: jbo
Differential Revision: https://reviews.freebsd.org/D51044

(cherry picked from commit d3c06bed2c16b434dd49958dee5de8c55ad00b85)