Starting with LLVM 8, clang does no longer use llvm-config to detect the
LLVM installation: https://github.com/llvm/llvm-project/commit/e4faa5c7986b7
Consequently, there is no point passing LLVM_CONFIG_PATH (in fact the
variable is unused currently).
This is already done for previous versions of clang which use
a release tarball, but must be done differently for the more
recent versions which use fetchFromGitHub.
Fixes clang-tools clangd wrapper