cmake: Fix install of libclang-cpp.so when LLVM_INSTALL_TOOLCHAIN_ONLY=ON

Summary:
If CLANG_LINK_CLANG_DYLIB is also enabled, then this library needs to be
installed.

Fixes PR42575.

Reviewers: beanz, smeenai

Subscribers: mgorny, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D64582

Conflicts:
	clang/tools/clang-shlib/CMakeLists.txt

llvm-svn: 365905