[lldb] Fix cross compiling on macOS

[mstorsjo]

Summary

When cross compiling, a separate nested cmake is spawned, for building
host code generation tools such as lldb-tblgen.

When cross compiling on macOS, the nested native build would trigger
the lldb check for libc++, if testing is enabled (which it is by default).
(Even if LLDB_INCLUDE_TESTS=OFF is set on the main build, it has to
be passed separately in CROSS_TOOLCHAIN_FLAGS_NATIVE to reach the
nested build.)

Skip this check when building the host tools when cross compiling, as
the user won't try to run tests in that nested build.

Alternatively, we could consider disabling all the *_INCLUDE_TESTS
by default in the nested host tools build.

[JDevlieghere]

Thank you! LGTM.

lldb/test/CMakeLists.txt

if(TARGET clang)
   if(TARGET asan)
     add_lldb_test_dependency(asan)
   endif()
 
   if(TARGET tsan)
     add_lldb_test_dependency(tsan)
   endif()
 
-  if(APPLE)
+  if(APPLE AND NOT LLVM_TARGET_IS_CROSSCOMPILE_HOST)
     # FIXME: Standalone builds should import the cxx target as well.
     if(LLDB_BUILT_STANDALONE)
       # For now check that the include directory exists.
       set(cxx_dir "${LLVM_BINARY_DIR}/include/c++")
       if(NOT EXISTS ${cxx_dir})
         message(WARNING "LLDB test suite requires libc++ in llvm/projects/libcxx or an existing build symlinked to ${cxx_dir}")
       endif()
     else()