clang-format: Allow include of clangFormat.h in managed context

Including VirtualFileSystem.h in the clangFormat.h indirectly includes <atomic>.
This header is blocked when compiling with /clr.

Patch by Maxime Beaulieu

Differential Revision: http://reviews.llvm.org/D19064

llvm-svn: 266319
diff --git a/clang/lib/Format/Format.cpp b/clang/lib/Format/Format.cpp
index 37788df..cc11d3d 100644
--- a/clang/lib/Format/Format.cpp
+++ b/clang/lib/Format/Format.cpp
@@ -22,6 +22,7 @@
 #include "clang/Basic/Diagnostic.h"
 #include "clang/Basic/DiagnosticOptions.h"
 #include "clang/Basic/SourceManager.h"
+#include "clang/Basic/VirtualFileSystem.h"
 #include "clang/Lex/Lexer.h"
 #include "llvm/ADT/STLExtras.h"
 #include "llvm/Support/Allocator.h"