[clang] [Driver] Support multiple configuration files

Support specifying multiple configuration files via multiple `--config`
options.  When multiple files are specified, the options from subsequent
files are appended to the options from the initial file.

While at it, remove the incorrect assertion about CfgFileName being
non-empty.  It can be empty if `--config ""` is passed, and it makes
sense to report it as non-existing file rather than crash.

Differential Revision: https://reviews.llvm.org/D134270