Install libLLVM if needed with LLVM_INSTALL_TOOLCHAIN_ONLY

Summary:
When LLVM_LINK_LLVM_DYLIB is set, the libLLVM shared
library needs to be installed in the toolchain.  Without
this chanage LLVM_INSTALL_TOOLCHAIN_ONLY combined with
LLVM_LINK_LLVM_DYLIB results in a broken install.

Patch by Sam Clegg

Differential Revision: https://reviews.llvm.org/D24676

llvm-svn: 281763