tools/clangtool: improve building/testing

[dvyukov]

With cf20fe9 we can build codeseach clang tool with just make and system libraries.
Some follow up improvments:

• support the same for syz-declextract
• install required libs to syz-env container (we need clang 19 version of libclang-cpp-dev libclang-dev)
• update docs to use only this way of building (if we want to keep tools on a particular clang release, we can't build with clang HEAD anyway, and there does not seem to be any advantages in building with clang HEAD)
• update codesearch and declextract tests to build and use these tools always somehow (instead of accepting a binary, and running some tests optionally).

[dvyukov]

fyi @FlorentRevest

[dvyukov]

Stretch: link the tools into Go binaries using cgo (feasible?)