[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/FuzzerCorpus.h b/llvm/lib/Fuzzer/FuzzerCorpus.h
index bbe192f..88e83b32 100644
--- a/llvm/lib/Fuzzer/FuzzerCorpus.h
+++ b/llvm/lib/Fuzzer/FuzzerCorpus.h
@@ -12,14 +12,14 @@
 #ifndef LLVM_FUZZER_CORPUS
 #define LLVM_FUZZER_CORPUS
 
-#include <random>
-#include <unordered_set>
-
 #include "FuzzerDefs.h"
 #include "FuzzerIO.h"
 #include "FuzzerRandom.h"
 #include "FuzzerSHA1.h"
 #include "FuzzerTracePC.h"
+#include <numeric>
+#include <random>
+#include <unordered_set>
 
 namespace fuzzer {