CMake option to link Clang/extra tools with libclang

Hi!

There was recent discussion about LLVM_LINK_LLVM_DYLIB:BOOL, which
allow to link tools with single shared LLVM library.

I think will be good idea to have similar option for Clang/extra tools
which include already libclang dynamic library with a lot LLVM and
CLang symbols.

This make sense for both debug and optimize builds, since not every
tool is used in same frequency. For example, I use Clang, Clang-tidy
and Include What You Use, but not other extra tools. Probably each
tool should have individual setting to link statically or dynamically.

This will reduce installation size and storage demands, especially
when multiple Clang versions need to be kept or replicated.

I filed https://llvm.org/bugs/show_bug.cgi?id=23749 in past, but it
was not noticed.

Eugene.