[dfsan] move dfsan_flags.h to cc files

[stephan.yichao.zhao]

Summary

D101666 needs this change.

[morehouse]

LGTM

compiler-rt/lib/dfsan/dfsan.h

 // Private DFSan header.
 //===----------------------------------------------------------------------===//
 
 #ifndef DFSAN_H
 #define DFSAN_H
 
 #include "sanitizer_common/sanitizer_internal_defs.h"
 
-#include "dfsan_flags.h"
 #include "dfsan_platform.h"
 
 using __sanitizer::u16;
 using __sanitizer::u32;
 using __sanitizer::uptr;
 
 // Copy declarations from public sanitizer/dfsan_interface.h header here.
 typedef u16 dfsan_label;

compiler-rt/lib/dfsan/dfsan_custom.cpp

 #include <sys/stat.h>
 #include <sys/time.h>
 #include <sys/types.h>
 #include <time.h>
 #include <unistd.h>
 
 #include "dfsan/dfsan.h"
 #include "dfsan/dfsan_chained_origin_depot.h"
+#include "dfsan/dfsan_flags.h"
 #include "dfsan/dfsan_thread.h"
 #include "sanitizer_common/sanitizer_common.h"
 #include "sanitizer_common/sanitizer_internal_defs.h"
 #include "sanitizer_common/sanitizer_linux.h"
 #include "sanitizer_common/sanitizer_stackdepot.h"
 
 using namespace __dfsan;