The clang importer has to deal with two virtual file systems, one coming
from clang, and one coming from swift. Currently, if both are set, we
emit a diagnostic that we'll pick the swift one.
This commit changes that, by merging the two virtual file systems into a
single overlay file system, and using that. To make this possible, we
always initialize the file manager with an overlay file system. In the
clang importer, we then create a new overlay file system, starting with
the one coming from clang, and adding overlays from swift on top.
The motivation for this change is the reproducer infrastructure in LLDB,
which adds a third virtual file system to the mix.
