[CrashReproducer] Add support for merging -ivfsoverlay

Merge all VFS mapped files inside -ivfsoverlay inputs into the vfs
overlay provided by the crash reproducer. This is the last missing piece
to allow crash reproducers to fully work with user frameworks; when
combined with headermaps, it allows clang to find additional frameworks.

rdar://problem/27913709

llvm-svn: 290326