[gn] Move Features.inc to clangd, create a config for it

ClangdLSPServer and clangd unittests now include Features.inc so we
need to append the target_gen_dir that contains it to their
include_dirs. To do so, we use a public config that's applied to
any target that depends on the features one.

Differential Revision: https://reviews.llvm.org/D60919

llvm-svn: 358837