[VFS] Add back setFallthrough for downstream users

This fixes lldb's build. We can remove this in the future if we want but
for now this will be nicer to existing consumers.

Author: bnbarham

llvm/include/llvm/Support/VirtualFileSystem.h

@@ -885,6 +885,10 @@ class RedirectingFileSystem : public vfs::FileSystem {
 
   StringRef getExternalContentsPrefixDir() const;
 
+  /// Sets the redirection kind to \c Fallthrough if true or \c RedirectOnly
+  /// otherwise. Will removed in the future, use \c setRedirection instead.
+  void setFallthrough(bool Fallthrough);
+
   void setRedirection(RedirectingFileSystem::RedirectKind Kind);
 
   std::vector<llvm::StringRef> getRoots() const;

llvm/lib/Support/VirtualFileSystem.cpp

@@ -1343,6 +1343,14 @@ StringRef RedirectingFileSystem::getExternalContentsPrefixDir() const {
   return ExternalContentsPrefixDir;
 }
 
+void RedirectingFileSystem::setFallthrough(bool Fallthrough) {
+  if (Fallthrough) {
+    Redirection = RedirectingFileSystem::RedirectKind::Fallthrough;
+  } else {
+    Redirection = RedirectingFileSystem::RedirectKind::RedirectOnly;
+  }
+}
+
 void RedirectingFileSystem::setRedirection(
     RedirectingFileSystem::RedirectKind Kind) {
   Redirection = Kind;