[llvm-profdata] Allow omission of -o for --text output

[MaskRay]

Summary

This makes it more convenient to get a text format profile.

Add an error for printing non-text format output to a terminal for instrumentation profile.
(It cannot be portably tested. For sample profile, raw_fd_ostream is hidden deeply so it's inconvenient to add a diagnostic.)

[davidxl]

lgtm

llvm/test/tools/llvm-profdata/text-dump.test

 Basic tests for testing text dump functions.
 
 RUN: llvm-profdata show --all-functions -counts --text %p/Inputs/basic.proftext > %t-basic.proftext1
 RUN: llvm-profdata merge -o %t-basic.proftext2 --text  %p/Inputs/basic.proftext
+RUN: llvm-profdata merge --text %p/Inputs/basic.proftext | diff %t-basic.proftext2 -
 
 RUN: llvm-profdata merge -binary -o %t-basic.profdata1 %t-basic.proftext1
 RUN: llvm-profdata merge -o %t-basic.profdata2 %t-basic.proftext2
 
 RUN: llvm-profdata show --all-functions -counts  %t-basic.profdata1 > %t-basic.dump3
 RUN: llvm-profdata show --all-functions -counts  %t-basic.profdata2 > %t-basic.dump4
 
 RUN: llvm-profdata merge -text -o %t-basic.proftext5 %t-basic.profdata1
 RUN: llvm-profdata merge -text -o %t-basic.proftext6 %t-basic.profdata2
 
 RUN: diff %t-basic.dump3 %t-basic.dump4
 RUN: diff %t-basic.proftext5 %t-basic.proftext6
 
 RUN: not llvm-profdata merge -gcc -o %t-basic-profdata3 %t-basic.proftext2 2>&1 | FileCheck %s --check-prefix=UNKNOWN
 UNKNOWN: unknown format is specified

llvm/tools/llvm-profdata/llvm-profdata.cpp

raw_fd_ostream Output(OutputFilename.data(), EC,
                                                 : sys::fs::OF_None);
   if (EC)
     exitWithErrorCode(EC, OutputFilename);
 
   if (OutputFormat == PF_Text) {
     if (Error E = Writer.writeText(Output))
       warn(std::move(E));
   } else {
+    if (Output.is_displayed())
+      exitWithError("cannot write a non-text format profile to the terminal");
     if (Error E = Writer.write(Output))
       warn(std::move(E));
   }
 }
 
 static void mergeInstrProfile(const WeightedFileVector &Inputs,
                               SymbolRemapper *Remapper,
                               StringRef OutputFilename,
                               ProfileFormat OutputFormat, bool OutputSparse,
                               unsigned NumThreads, FailureMode FailMode) {
-  if (OutputFilename.compare("-") == 0)
-    exitWithError("cannot write indexed profdata format to stdout");
-
   if (OutputFormat != PF_Binary && OutputFormat != PF_Compact_Binary &&
       OutputFormat != PF_Ext_Binary && OutputFormat != PF_Text)
     exitWithError("unknown format is specified");
 
   std::mutex ErrorLock;
   SmallSet<instrprof_error, 4> WriterErrorCodes;
 
   // If NumThreads is not specified, auto-detect a good default.
static int merge_main(int argc, const char *argv[]) {
   cl::opt<bool> DumpInputFileList(
       "dump-input-file-list", cl::init(false), cl::Hidden,
       cl::desc("Dump the list of input files and their weights, then exit"));
   cl::opt<std::string> RemappingFile("remapping-file", cl::value_desc("file"),
                                      cl::desc("Symbol remapping file"));
   cl::alias RemappingFileA("r", cl::desc("Alias for --remapping-file"),
                            cl::aliasopt(RemappingFile));
   cl::opt<std::string> OutputFilename("output", cl::value_desc("output"),
-                                      cl::init("-"), cl::Required,
-                                      cl::desc("Output file"));
+                                      cl::init("-"), cl::desc("Output file"));
   cl::alias OutputFilenameA("o", cl::desc("Alias for --output"),
                             cl::aliasopt(OutputFilename));
   cl::opt<ProfileKinds> ProfileKind(
       cl::desc("Profile kind:"), cl::init(instr),
       cl::values(clEnumVal(instr, "Instrumentation profile (default)"),
                  clEnumVal(sample, "Sample profile")));
   cl::opt<ProfileFormat> OutputFormat(
       cl::desc("Format of output profile"), cl::init(PF_Binary),