Enable -fsanitize=fuzzer and -fsanitize=fuzzer-no-link on Windows.

[metzman]

Summary

This allows -fsanitize=fuzzer and -fsanitize=fuzzer-no-link for the MSVC toolchain (Windows).
It puts arrays for storing the PC table, 8 bit counters, and coverage guards, in their own sections, so their sizes can be determined (since ___start isn't supported by linkers targeting MSVC toolchain).

[metzman]

Matt, could you please take a look at this?

This patch puts each array used by sancov/libFuzzer into a unique section.
The section names all begin with ".SCOV", the next letter in the name signifies what the section is used for (eg: in SCOVC$M, C stands for Counters).
The section names all end in $M so that libFuzzer will know the size and location of the arrays (this is how symbols such as __start___sancov_cntrs are set by the linker when targeting MSVC. This technique was first used for libFuzzer here)
This change needs my patch for compiler-rt (specifically this file) to make sense.

lib/Transforms/Instrumentation/SanitizerCoverage.cpp

813

This naming scheme isn't great, but we need 3 unique names that are less than or equal to 8 chars (this the maximum section name length, see https://msdn.microsoft.com/en-us/library/7977wcck.aspx).

814

Maybe I shouldn't rename the section used to store the guard array, since I am planning on leaving it in a broken state as per (what I think are) kcc's wishes, but I want to be consistent with the other section names.
What do you think?

[kcc]

is a lit test possible here?

[metzman]

@kcc I'm abandoning this review and resubmitting this patch as part of [[ URL | https://reviews.llvm.org/D51022 ]] review, so that my changes to clang, llvm, and compiler-rt can be seen together where they make sense.

I added a lit test there.

lib/Transforms/Instrumentation/SanitizerCoverage.cpp

if (Options.StackDepth && IsEntryBB && !IsLeafFunc) {
     auto Store = ThenIRB.CreateStore(FrameAddrInt, SanCovLowestStack);
     SetNoSanitizeMetadata(LowestStack);
     SetNoSanitizeMetadata(Store);
   }
 }
 
 std::string
 SanitizerCoverageModule::getSectionName(const std::string &Section) const {
-  if (TargetTriple.getObjectFormat() == Triple::COFF)
-    return ".SCOV$M";
+  if (TargetTriple.getObjectFormat() == Triple::COFF) {
+    if (Section == SanCovCountersSectionName)
+      return ".SCOVC$M";
+    else if (Section == SanCovPCsSectionName)
+      return ".SCOVP$M";
+    return ".SCOVG$M";  // For SanCovGuardsSectionName.
+  }
   if (TargetTriple.isOSBinFormatMachO())
     return "__DATA,__" + Section;
   return "__" + Section;
 }
 
 std::string
 SanitizerCoverageModule::getSectionStart(const std::string &Section) const {
   if (TargetTriple.isOSBinFormatMachO())