diff --git a/llvm/include/llvm/Support/VirtualFileSystem.h b/llvm/include/llvm/Support/VirtualFileSystem.h
--- a/llvm/include/llvm/Support/VirtualFileSystem.h
+++ b/llvm/include/llvm/Support/VirtualFileSystem.h
@@ -731,6 +731,11 @@
          SourceMgr::DiagHandlerTy DiagHandler, StringRef YAMLFilePath,
          void *DiagContext, IntrusiveRefCntPtr<FileSystem> ExternalFS);
 
+  /// Redirect each of the remapped files from first to second.
+  static std::unique_ptr<RedirectingFileSystem>
+  create(ArrayRef<std::pair<std::string, std::string>> RemappedFiles,
+         bool UseExternalNames, FileSystem &ExternalFS);
+
   ErrorOr<Status> status(const Twine &Path) override;
   ErrorOr<std::unique_ptr<File>> openFileForRead(const Twine &Path) override;
 
diff --git a/llvm/lib/Support/VirtualFileSystem.cpp b/llvm/lib/Support/VirtualFileSystem.cpp
--- a/llvm/lib/Support/VirtualFileSystem.cpp
+++ b/llvm/lib/Support/VirtualFileSystem.cpp
@@ -1272,7 +1272,8 @@
     return true;
   }
 
-  RedirectingFileSystem::Entry *
+public:
+  static RedirectingFileSystem::Entry *
   lookupOrCreateEntry(RedirectingFileSystem *FS, StringRef Name,
                       RedirectingFileSystem::Entry *ParentEntry = nullptr) {
     if (!ParentEntry) { // Look for a existent root
@@ -1314,6 +1315,7 @@
     return DE->getLastContent();
   }
 
+private:
   void uniqueOverlayTree(RedirectingFileSystem *FS,
                          RedirectingFileSystem::Entry *SrcE,
                          RedirectingFileSystem::Entry *NewParentE = nullptr) {
@@ -1682,6 +1684,61 @@
   return FS;
 }
 
+std::unique_ptr<RedirectingFileSystem> RedirectingFileSystem::create(
+    ArrayRef<std::pair<std::string, std::string>> RemappedFiles,
+    bool UseExternalNames, FileSystem &ExternalFS) {
+  std::unique_ptr<RedirectingFileSystem> FS(
+      new RedirectingFileSystem(&ExternalFS));
+  FS->UseExternalNames = UseExternalNames;
+
+  StringMap<RedirectingFileSystem::Entry *> Entries;
+
+  for (auto &Mapping : llvm::reverse(RemappedFiles)) {
+    SmallString<128> From = StringRef(Mapping.first);
+    SmallString<128> To = StringRef(Mapping.second);
+    {
+      auto EC = ExternalFS.makeAbsolute(From);
+      (void)EC;
+      assert(!EC && "Could not make absolute path");
+    }
+
+    // Check if we've already mapped this file. The first one we see (in the
+    // reverse iteration) wins.
+    RedirectingFileSystem::Entry *&ToEntry = Entries[From];
+    if (ToEntry)
+      continue;
+
+    // Add parent directories.
+    RedirectingFileSystem::Entry *Parent = nullptr;
+    StringRef FromDirectory = llvm::sys::path::parent_path(From);
+    for (auto I = llvm::sys::path::begin(FromDirectory),
+              E = llvm::sys::path::end(FromDirectory);
+         I != E; ++I) {
+      Parent = RedirectingFileSystemParser::lookupOrCreateEntry(FS.get(), *I,
+                                                                Parent);
+    }
+    assert(Parent && "File without a directory?");
+    {
+      auto EC = ExternalFS.makeAbsolute(To);
+      (void)EC;
+      assert(!EC && "Could not make absolute path");
+    }
+
+    // Add the file.
+    auto NewFile =
+        std::make_unique<RedirectingFileSystem::RedirectingFileEntry>(
+            llvm::sys::path::filename(From), To,
+            UseExternalNames
+                ? RedirectingFileSystem::RedirectingFileEntry::NK_External
+                : RedirectingFileSystem::RedirectingFileEntry::NK_Virtual);
+    ToEntry = NewFile.get();
+    cast<RedirectingFileSystem::RedirectingDirectoryEntry>(Parent)->addContent(
+        std::move(NewFile));
+  }
+
+  return FS;
+}
+
 ErrorOr<RedirectingFileSystem::Entry *>
 RedirectingFileSystem::lookupPath(const Twine &Path_) const {
   SmallString<256> Path;
diff --git a/llvm/unittests/Support/VirtualFileSystemTest.cpp b/llvm/unittests/Support/VirtualFileSystemTest.cpp
--- a/llvm/unittests/Support/VirtualFileSystemTest.cpp
+++ b/llvm/unittests/Support/VirtualFileSystemTest.cpp
@@ -2287,3 +2287,89 @@
   EXPECT_FALSE(FS->exists(_b.path("b")));
   EXPECT_FALSE(FS->exists(_c.path("c")));
 }
+
+TEST(VFSFromRemappedFilesTest, Basic) {
+  IntrusiveRefCntPtr<vfs::InMemoryFileSystem> BaseFS =
+      new vfs::InMemoryFileSystem;
+  BaseFS->addFile("//root/b", 0, MemoryBuffer::getMemBuffer("contents of b"));
+  BaseFS->addFile("//root/c", 0, MemoryBuffer::getMemBuffer("contents of c"));
+
+  std::vector<std::pair<std::string, std::string>> RemappedFiles = {
+      {"//root/a/a", "//root/b"},
+      {"//root/a/b/c", "//root/c"},
+  };
+  auto RemappedFS = vfs::RedirectingFileSystem::create(
+      RemappedFiles, /*UseExternalNames=*/false, *BaseFS);
+
+  auto StatA = RemappedFS->status("//root/a/a");
+  auto StatB = RemappedFS->status("//root/a/b/c");
+  ASSERT_TRUE(StatA);
+  ASSERT_TRUE(StatB);
+  EXPECT_EQ("//root/a/a", StatA->getName());
+  EXPECT_EQ("//root/a/b/c", StatB->getName());
+
+  auto BufferA = RemappedFS->getBufferForFile("//root/a/a");
+  auto BufferB = RemappedFS->getBufferForFile("//root/a/b/c");
+  ASSERT_TRUE(BufferA);
+  ASSERT_TRUE(BufferB);
+  EXPECT_EQ("contents of b", (*BufferA)->getBuffer());
+  EXPECT_EQ("contents of c", (*BufferB)->getBuffer());
+}
+
+TEST(VFSFromRemappedFilesTest, UseExternalNames) {
+  IntrusiveRefCntPtr<vfs::InMemoryFileSystem> BaseFS =
+      new vfs::InMemoryFileSystem;
+  BaseFS->addFile("//root/b", 0, MemoryBuffer::getMemBuffer("contents of b"));
+  BaseFS->addFile("//root/c", 0, MemoryBuffer::getMemBuffer("contents of c"));
+
+  std::vector<std::pair<std::string, std::string>> RemappedFiles = {
+      {"//root/a/a", "//root/b"},
+      {"//root/a/b/c", "//root/c"},
+  };
+  auto RemappedFS = vfs::RedirectingFileSystem::create(
+      RemappedFiles, /*UseExternalNames=*/true, *BaseFS);
+
+  auto StatA = RemappedFS->status("//root/a/a");
+  auto StatB = RemappedFS->status("//root/a/b/c");
+  ASSERT_TRUE(StatA);
+  ASSERT_TRUE(StatB);
+  EXPECT_EQ("//root/b", StatA->getName());
+  EXPECT_EQ("//root/c", StatB->getName());
+
+  auto BufferA = RemappedFS->getBufferForFile("//root/a/a");
+  auto BufferB = RemappedFS->getBufferForFile("//root/a/b/c");
+  ASSERT_TRUE(BufferA);
+  ASSERT_TRUE(BufferB);
+  EXPECT_EQ("contents of b", (*BufferA)->getBuffer());
+  EXPECT_EQ("contents of c", (*BufferB)->getBuffer());
+}
+
+TEST(VFSFromRemappedFilesTest, LastMappingWins) {
+  IntrusiveRefCntPtr<vfs::InMemoryFileSystem> BaseFS =
+      new vfs::InMemoryFileSystem;
+  BaseFS->addFile("//root/b", 0, MemoryBuffer::getMemBuffer("contents of b"));
+  BaseFS->addFile("//root/c", 0, MemoryBuffer::getMemBuffer("contents of c"));
+
+  std::vector<std::pair<std::string, std::string>> RemappedFiles = {
+      {"//root/a", "//root/b"},
+      {"//root/a", "//root/c"},
+  };
+  auto RemappedFSKeepName = vfs::RedirectingFileSystem::create(
+      RemappedFiles, /*UseExternalNames=*/false, *BaseFS);
+  auto RemappedFSExternalName = vfs::RedirectingFileSystem::create(
+      RemappedFiles, /*UseExternalNames=*/true, *BaseFS);
+
+  auto StatKeepA = RemappedFSKeepName->status("//root/a");
+  auto StatExternalA = RemappedFSExternalName->status("//root/a");
+  ASSERT_TRUE(StatKeepA);
+  ASSERT_TRUE(StatExternalA);
+  EXPECT_EQ("//root/a", StatKeepA->getName());
+  EXPECT_EQ("//root/c", StatExternalA->getName());
+
+  auto BufferKeepA = RemappedFSKeepName->getBufferForFile("//root/a");
+  auto BufferExternalA = RemappedFSExternalName->getBufferForFile("//root/a");
+  ASSERT_TRUE(BufferKeepA);
+  ASSERT_TRUE(BufferExternalA);
+  EXPECT_EQ("contents of c", (*BufferKeepA)->getBuffer());
+  EXPECT_EQ("contents of c", (*BufferExternalA)->getBuffer());
+}