Don't crash sancov if file is unreadable.

[vitalybuka]

Summary

Caller can provides the list of .so files where some files are
unreadable (e.g linux-vdso.so.1). It's more convenient to handler this in
sancov with warning then making all callers to check files.

[aizatsky]

LGTM

[vitalybuka]

Mike, could you please also commit this patch?

tools/sancov/sancov.cc

 static cl::opt<std::string>
     ClBlacklist("blacklist", cl::init(""),
                 cl::desc("Blacklist file (sanitizer blacklist format)."));
 
 static cl::opt<bool> ClUseDefaultBlacklist(
     "use_default_blacklist", cl::init(true), cl::Hidden,
     cl::desc("Controls if default blacklist should be used."));
 
-static const char *const DefaultBlacklistStr = 
-      "fun:__sanitizer_.*\n"
+static const char *const DefaultBlacklistStr = "fun:__sanitizer_.*\n"
                                                "src:/usr/include/.*\n"
                                                "src:.*/libc\\+\\+/.*\n";
 
 // --------- FORMAT SPECIFICATION ---------
 
 struct FileHeader {
   uint32_t Bitness;
   uint32_t Magic;
 };
 
for (char Ch : S) {
     }
   }
   return Result;
 }
 
 static ErrorOr<bool> isCoverageFile(std::string FileName) {
   ErrorOr<std::unique_ptr<MemoryBuffer>> BufOrErr =
       MemoryBuffer::getFile(FileName);
-  if (!BufOrErr)
+  if (!BufOrErr) {
+    errs() << "Warning: " << BufOrErr.getError().message() << "("
+           << BufOrErr.getError().value()
+           << "), filename: " << llvm::sys::path::filename(FileName) << "\n";
     return BufOrErr.getError();
+  }
   std::unique_ptr<MemoryBuffer> Buf = std::move(BufOrErr.get());
   if (Buf->getBufferSize() < 8) {
     return false;
   }
   const FileHeader *Header =
       reinterpret_cast<const FileHeader *>(Buf->getBufferStart());
   return Header->Magic == BinCoverageMagic;
 }
readCmdArguments(std::vector<std::string> FileNames) {
     // Short name => file name.
     std::map<std::string, std::string> ObjFiles;
     std::string FirstObjFile;
     std::set<std::string> CovFiles;
 
     // Partition input values into coverage/object files.
     for (const auto &FileName : FileNames) {
       auto ErrorOrIsCoverage = isCoverageFile(FileName);
-      FailIfError(ErrorOrIsCoverage);
+      if (!ErrorOrIsCoverage)
+        continue;
       if (ErrorOrIsCoverage.get()) {
         CovFiles.insert(FileName);
       } else {
         auto ShortFileName = llvm::sys::path::filename(FileName);
         if (ObjFiles.find(ShortFileName) != ObjFiles.end()) {
           Fail("Duplicate binary file with a short name: " + ShortFileName);
         }