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. The
motivation for this change is the reproducer infrastructure in LLDB,
which adds a third virtual file system to the mix.
(cherry picked from commit 94ef5431ff)
Adds the -vfsoverlay frontend option that enables the user to pass
VFS overlay YAML files to Swift. These files define a (potentially
many-layered) virtual mapping on which we predicate a VFS.
Switch all input-based memory buffer reads in the Frontend to the new
FileSystem-based approach.
