Lift VFS from clang to llvm (NFC)

This patch moves the virtual file system form clang to llvm so it can be
used by more projects.

Concretely the patch:
 - Moves VirtualFileSystem.{h|cpp} from clang/Basic to llvm/Support.
 - Moves the corresponding unit test from clang to llvm.
 - Moves the vfs namespace from clang::vfs to llvm::vfs.
 - Formats the lines affected by this change, mostly this is the result of
   the added llvm namespace.

RFC on the mailing list:
http://lists.llvm.org/pipermail/llvm-dev/2018-October/126657.html

Differential revision: https://reviews.llvm.org/D52783

llvm-svn: 344140

clang/lib/Serialization/ModuleManager.cpp

@@ -16,7 +16,6 @@
 #include "clang/Basic/FileManager.h"
 #include "clang/Basic/LLVM.h"
 #include "clang/Basic/MemoryBufferCache.h"
-#include "clang/Basic/VirtualFileSystem.h"
 #include "clang/Frontend/PCHContainerOperations.h"
 #include "clang/Lex/HeaderSearch.h"
 #include "clang/Lex/ModuleMap.h"
@@ -33,6 +32,7 @@
 #include "llvm/Support/ErrorOr.h"
 #include "llvm/Support/GraphWriter.h"
 #include "llvm/Support/MemoryBuffer.h"
+#include "llvm/Support/VirtualFileSystem.h"
 #include <algorithm>
 #include <cassert>
 #include <memory>
@@ -150,7 +150,7 @@ ModuleManager::addModule(StringRef FileName, ModuleKind Type,
 
   if (NewModule->Kind == MK_ImplicitModule) {
     std::string TimestampFilename = NewModule->getTimestampFilename();
-    vfs::Status Status;
+    llvm::vfs::Status Status;
     // A cached stat value would be fine as well.
     if (!FileMgr.getNoncachedStatValue(TimestampFilename, Status))
       NewModule->InputFilesValidationTimestamp =