[Driver] Enable nested configuration files

Users may partition parameters specified by configuration file and put
different groups into separate files. These files are inserted into the
main file using constructs `@file`. Relative file names in it are
resolved relative to the including configuration file and this is not
convenient in some cases. A configuration file, which resides in system
directory, may need to include a file with user-defined parameters and
still provide default definitions if such file is absent.

To solve such problems, the option `--config=` is allowed inside
configuration files. Like `@file` it results in insertion of
command-line arguments but the algorithm of file search is different and
allows overriding system definitions with user ones.

Differential Revision: https://reviews.llvm.org/D136354