[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

git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@350737 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed