Driver: Accept multiple --config options if filenames are the same

Summary:
We're trying to use the --config options to pass distro specific
options for Fedora via the CFLAGS variable.  However, some projects
end up using the CFLAGS variable multiple times in their command line,
which leads to an error when --config is used.

This patch resolves this issue by allowing more than one --config option
on the command line as long as the file names are the same.

Reviewers: sepavloff, hfinkel

Reviewed By: sepavloff

Subscribers: cfe-commits, llvm-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D81424