[CMake] Workaround for PR24144: avoid installing libcxx_tsan and libcxx_msan...

[CMake] Workaround for PR24144: avoid installing libcxx_tsan and libcxx_msan by default "ninja install" command.

Summary:
Exclude external libc++ builds from "all" target, so that they are only
build on demand, and are not installed together with LLVM/Clang.

Reviewers: hans

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D11252

llvm-svn: 242424