[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/FuzzerDefs.h b/llvm/lib/Fuzzer/FuzzerDefs.h
index 89d675a..a699045 100644
--- a/llvm/lib/Fuzzer/FuzzerDefs.h
+++ b/llvm/lib/Fuzzer/FuzzerDefs.h
@@ -8,6 +8,7 @@
 //===----------------------------------------------------------------------===//
 // Basic definitions.
 //===----------------------------------------------------------------------===//
+
 #ifndef LLVM_FUZZER_DEFS_H
 #define LLVM_FUZZER_DEFS_H
 
@@ -77,4 +78,5 @@
 inline uint64_t Bswap(uint64_t x) { return __builtin_bswap64(x); }
 
 }  // namespace fuzzer
+
 #endif  // LLVM_FUZZER_DEFS_H