When creating a standalone build of the runtimes sub-project, the
current CMake implementation looks for a lit executable that might
potentially exist in the build tree and unconditionally overrides the
value of `LLVM_EXTERNAL_LIT`. Due to this, any value passed via
`-DLLVM_EXTERNAL_LIT` when configuring the CMake project is ignored.
This change adds the `ALLOW_EXTERNAL` argument to the
`get_llvm_lit_path` call in the runtimes' CMakeLists.txt, allowing any
value previously set to be considered.
6fcdde2a4e