[libFuzzer] Clean up headers and file formatting of LibFuzzer files.

Reorganize #includes to follow LLVM Coding Standards.
Include some missing headers. Required to use `Printf()`.

Aside from that, this patch contains no functional change.
It is purely a re-organization.

Differential Revision: https://reviews.llvm.org/D27363

llvm-svn: 289560
diff --git a/llvm/lib/Fuzzer/FuzzerUtilDarwin.cpp b/llvm/lib/Fuzzer/FuzzerUtilDarwin.cpp
index 62f0f22..9674368 100644
--- a/llvm/lib/Fuzzer/FuzzerUtilDarwin.cpp
+++ b/llvm/lib/Fuzzer/FuzzerUtilDarwin.cpp
@@ -10,6 +10,7 @@
 //===----------------------------------------------------------------------===//
 #include "FuzzerDefs.h"
 #if LIBFUZZER_APPLE
+
 #include "FuzzerIO.h"
 #include <mutex>
 #include <signal.h>
@@ -145,5 +146,7 @@
   }
   return ProcessStatus;
 }
-}
+
+} // namespace fuzzer
+
 #endif // LIBFUZZER_APPLE