Gentoo's Bugzilla –
Attachment 638846 Details for
Bug 722978sys-devel/llvm fails to build with Clang with -rtlib=compiler-rt (default with sys-devel/clang[default-compiler-rt]) (except with -stdlib=libc++ and sys-libs/llvm-libunwind transitively used through sys-libs/libcxx{,abi}[libunwind])