[clangd] Fix a regression issue caused by r348365.

Summary:
With r348365, we now detect libc++ dir using the actual compiler path
(from the compilation command), rather than the resource-dir.

This new behavior will cause clangd couldn't find libc++ dir (even the libc++ is
built from the source) when using a fallback compilation command (`clang xxx`)

The fix is to use `<clangd_install_dir>/clang` as the actual compiler path.

Reviewers: ilya-biryukov

Reviewed By: ilya-biryukov

Subscribers: ioeric, MaskRay, jkorous, arphaman, kadircet, cfe-commits

Differential Revision: https://reviews.llvm.org/D56380

llvm-svn: 350515