To determine the file magic, the file needs to be opened and read.
This is done with *each* file in $LD_LIBRARY_PATH, including ./
If one of them is e.g. a FIFO then reading blocks until someone
writes into the FIFO, which might cause the process to hang. This
was reported a couple of times, such as at https://root-forum.cern.ch/t/compiling-from-source-first-interactive-command-hangs/43997/5

Solution: only check for the file magic of *regular* files.

Sadly, llvm::sys::fs::get_file_type never sets file_not_found but
returns an unspecific status_error.