[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

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@358837 91177308-0d34-0410-b5e6-96231b3b80d8
3 files changed