Add a driver option -ivfsoverlay

Reads the description of a virtual filesystem from a file and overlays
it over the real file system.

llvm-svn: 202176
diff --git a/clang/test/VFS/Inputs/actual_header.h b/clang/test/VFS/Inputs/actual_header.h
new file mode 100644
index 0000000..f61e586
--- /dev/null
+++ b/clang/test/VFS/Inputs/actual_header.h
@@ -0,0 +1 @@
+void bar(void);