[lldb] Make compiler-rt an optional LLDB test dependency

Make compiler-rt an LLDB test dependency if the corresponding target
exists. Similarly we already have `asan` and `tsan` as optional test
dependencies, but we need the `compiler-rt` target when enabling
compiler-rt trough LLVM_ENABLE_RUNTIMES.

lldb/test/CMakeLists.txt

@@ -103,6 +103,10 @@ if(TARGET clang)
     add_lldb_test_dependency(tsan)
   endif()
 
+  if (TARGET compiler-rt)
+    add_lldb_test_dependency(compiler-rt)
+  endif()
+
   if(APPLE AND NOT LLVM_TARGET_IS_CROSSCOMPILE_HOST)
     # FIXME: Standalone builds should import the cxx target as well.
     if(LLDB_BUILT_STANDALONE)