Add .clang-format and run clang-format on runtime/platform.

runtime/platform/c99_support_win.h

 // Copyright (c) 2011, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 #ifndef RUNTIME_PLATFORM_C99_SUPPORT_WIN_H_
 #define RUNTIME_PLATFORM_C99_SUPPORT_WIN_H_
 
 #if defined(_MSC_VER) && (_MSC_VER < 1800)
 
 // Before Visual Studio 2013 Visual C++ was missing a bunch of C99 math macros
 // and functions. Define them here.
 
 #include <float.h>
 #include <string.h>
 
 #include <cmath>
 
 static const unsigned __int64 kQuietNaNMask =
     static_cast<unsigned __int64>(0xfff) << 51;
 
 
 #ifndef va_copy
 #define va_copy(dst, src) (memmove(&(dst), &(src), sizeof(dst)))
-#endif  /* va_copy */
+#endif /* va_copy */
 
 
-#define NAN \
-    *reinterpret_cast<const double*>(&kQuietNaNMask)
+#define NAN *reinterpret_cast<const double*>(&kQuietNaNMask)
 
 namespace std {
 
 static inline int isinf(double x) {
   return (_fpclass(x) & (_FPCLASS_PINF | _FPCLASS_NINF)) != 0;
 }
 
 static inline int isnan(double x) {
   return _isnan(x);
 }
@@ skipping 34 matching lines @@
 }
 
 // Windows does not have strtoll defined.
 #if defined(_MSC_VER)
 #define strtoll _strtoi64
 #endif
 
 #endif
 
 #endif  // RUNTIME_PLATFORM_C99_SUPPORT_WIN_H_