[CMake] Fix standalone builds: workaround the cxx target not getting imported...

[CMake] Fix standalone builds: workaround the cxx target not getting imported yet (unlike clang target)

Summary: Handle standalone builds separately and print a warning if we have no libcxx.

Reviewers: aprantl, JDevlieghere

Reviewed By: JDevlieghere

Subscribers: mgorny, lldb-commits, #lldb

Differential Revision: https://reviews.llvm.org/D56399

llvm-svn: 350737