[BOLT] Add `-o` option to merge-fdata

[Amir]

Can you please add a test for the option, covering -o file, -o - (stdout), and no -o modes?
bolt/test/X86/merge-fdata-bat-mode.test is a good reference for such a test.

[Amir]

LGTM sans clang-format issue

bolt/tools/merge-fdata/merge-fdata.cpp

95

clang-format might complain about output variable name not being capitalized

bolt/tools/merge-fdata/merge-fdata.cpp

 
 static cl::opt<bool>
 SuppressMergedDataOutput("q",
   cl::desc("do not print merged data to stdout"),
   cl::init(false),
   cl::Optional,
   cl::cat(MergeFdataCategory));
 
+static cl::opt<std::string>
+OutputFilePath("o",
+  cl::value_desc("file"),
+  cl::desc("Write output to <file>"),
+  cl::cat(MergeFdataCategory));
+
 } // namespace opts
 
 namespace {
 
 static StringRef ToolName;
 
 static void report_error(StringRef Message, std::error_code EC) {
   assert(EC);
   errs() << ToolName << ": '" << Message << "': " << EC.message() << ".\n";
   exit(1);
 }
 
 static void report_error(Twine Message, StringRef CustomError) {
   errs() << ToolName << ": '" << Message << "': " << CustomError << ".\n";
   exit(1);
 }
 
+static raw_fd_ostream &output() {
+  if (opts::OutputFilePath.empty() || opts::OutputFilePath == "-") {
+    return outs();
+  } else {
+    std::error_code EC;
+    static raw_fd_ostream output(opts::OutputFilePath, EC);
+    if (EC)
+      report_error(opts::OutputFilePath, EC);
+    return output;
+  }
+}
+
 void mergeProfileHeaders(BinaryProfileHeader &MergedHeader,
                          const BinaryProfileHeader &Header) {
   if (MergedHeader.FileName.empty())
     MergedHeader.FileName = Header.FileName;
 
   if (!MergedHeader.FileName.empty() &&
       MergedHeader.FileName != Header.FileName)
     errs() << "WARNING: merging profile from a binary for " << Header.FileName
for (StringRef Line : Lines) {
         report_error(Filename, "Malformed / corrupted profile counter");
       Count += Entries.lookup(Signature);
       Entries.insert_or_assign(Signature, Count);
     }
     First = false;
   }
 
   if (BoltedCollection)
-    outs() << "boltedcollection\n";
+    output() << "boltedcollection\n";
   for (const auto &Entry : Entries)
-    outs() << Entry.getKey() << " " << Entry.getValue() << "\n";
+    output() << Entry.getKey() << " " << Entry.getValue() << "\n";
 
   errs() << "Profile from " << Filenames.size() << " files merged.\n";
 }
 
 } // anonymous namespace
 
 int main(int argc, char **argv) {
   // Print a stack trace if we signal out.
for (BinaryFunctionProfile &BF : BP.Functions) {
       }
 
       BinaryFunctionProfile &MergedBF = MergedBFs.find(BF.Name)->second;
       mergeFunctionProfile(MergedBF, std::move(BF));
     }
   }
 
   if (!opts::SuppressMergedDataOutput) {
-    yaml::Output YamlOut(outs());
+    yaml::Output YamlOut(output());
 
     BinaryProfile MergedProfile;
     MergedProfile.Header = MergedHeader;
     MergedProfile.Functions.resize(MergedBFs.size());
     std::transform(
         MergedBFs.begin(), MergedBFs.end(), MergedProfile.Functions.begin(),
         [](StringMapEntry<BinaryFunctionProfile> &V) { return V.second; });