[lldb][cmake] Allow specifying custom libcxx for tests in standalone builds

Standalone builds currently do not set the `LLDB_HAS_LIBCXX`,
`LIBCXX_LIBRARY_DIR`, `LIBCXX_GENERATED_INCLUDE_DIR`.
These are necessary for API tests with `USE_LIBCPP` to run against
a custom built libcxx. Thus on all buildbots using standalone builds
(most notably the public swift-ci), the API tests always run against
the libcxx headers in the system SDK.

This patch introduces a new cmake variable `LLDB_TEST_LIBCXX_ROOT_DIR`
that allows us to point the tests in standalone builds to a custom
libcxx directory.

Since the user can control the libcxx location we can hard error if
no such custom libcxx build exists.

Differential Revision: https://reviews.llvm.org/D150954