Hello, seaofash!
You can do this to use lldb in sys-devel/llvm-3.9.1-r1:
# mkdir -p /etc/portage/profile/use.mask
# echo '-lldb' > /etc/portage/profile/use.mask/test-forum
# USE="lldb" emerge -1 =sys-devel/llvm-3.9.1-r1 or # echo '=sys-devel/llvm-3.9.1-r1 lldb' > /etc/portage/package.use/test-forum
But if you want llvm, lldb and clang in version 4, you need all this:
# mkdir -p /var/git/overlay/local/sys-devel/llvm
# cd /var/git/overlay/local/sys-devel/llvm/
# wget https://gitweb.gentoo.org/repo/gentoo.git/plain/sys-devel/llvm/llvm-4.0.1.ebuild
# ebuild llvm-4.0.1.ebuild digest
# mkdir -p /var/git/overlay/local/sys-devel/llvm-common
# cd /var/git/overlay/local/sys-devel/llvm-common/
# wget https://gitweb.gentoo.org/repo/gentoo.git/plain/sys-devel/llvm-common/llvm-common-4.0.1.ebuild
# ebuild llvm-common-4.0.1.ebuild digest
And the file /etc/portage/package.unmask/test-forum with this:
sys-devel/clang:4
sys-devel/llvm:4
=dev-util/lldb-4.0.1
=sys-devel/llvm-common-4.0.1
=sys-devel/clang-runtime-4.0.1