Declare that the DIRECTORY root is an instance of CLASS.
DIRECTORY is the name of a directory, a string.
CLASS is the name of a project class, a symbol.
MTIME is either the modification time of the directory-local
variables file that defined this class, or nil.
When a file beneath DIRECTORY is visited, the mode-specific
variables from CLASS are applied to the buffer. The variables
for a class are defined using `dir-locals-set-class-variables'.