[libFuzzer] Update implementation of sanitizer coverage to work when guards are initialized to 0xffffffff.

[mpividori]

Summary

Update sanitizer coverage implementation according to the changes mentioned in: https://reviews.llvm.org/D28436

At this point, I am finally able to compile all libFuzzer tests for Windows, and this is the result:

Expected Passes    : 48
Unsupported Tests  : 2
Unexpected Failures: 27

Thanks,
Marcos

lib/Fuzzer/FuzzerTracePC.cpp

size_t TracePC::GetTotalPCCoverage() {
   size_t Res = 0;
   for (size_t i = 1; i < GetNumPCs(); i++)
     if (PCs[i])
       Res++;
   return Res;
 }
 
 void TracePC::HandleInit(uint32_t *Start, uint32_t *Stop) {
-  if (Start == Stop || *Start) return;
+  if (Start == Stop) return;
   assert(NumModules < sizeof(Modules) / sizeof(Modules[0]));
   for (uint32_t *P = Start; P < Stop; P++)
+    // Ignore zero padding.
+    if (*P) {
+      if (*P != 0xffffffff) // This module was initialized before.
+        return;
       *P = ++NumGuards;
+    }
   Modules[NumModules].Start = Start;
   Modules[NumModules].Stop = Stop;
   NumModules++;
 }
 
 void TracePC::PrintModuleInfo() {
   Printf("INFO: Loaded %zd modules (%zd guards): ", NumModules, NumGuards);
   for (size_t i = 0; i < NumModules; i++)