[dsymutil][NFC} Rename thread related command line options

[JDevlieghere]

Summary

This makes the command line options consistent with llvm-cov and
llvm-profdata, which both use -num-threads and -j.

This also addresses the conflict reported after landing D39355.

llvm/trunk/test/tools/dsymutil/cmdline.test

 RUN: llvm-dsymutil -help 2>&1 | FileCheck --check-prefix=HELP %s
 HELP: OVERVIEW: manipulate archived DWARF debug symbol files.
 HELP: USAGE: llvm-dsymutil{{[^ ]*}} [options] <input files>
 HELP-NOT: -reverse-iterate
 HELP: Specific Options:
 HELP: -arch=<string>
 HELP: -dump-debug-map
 HELP: -flat
 HELP: -no-odr
 HELP: -no-output
 HELP: -no-swiftmodule-timestamp
+HELP: -num-threads=<n>
 HELP: -o=<filename>
 HELP: -oso-prepend-path=<path>
 HELP: -symtab
-HELP: -threads=<n>
 HELP: -verbose
 HELP: -y
 HELP-NOT: -reverse-iterate
 
 RUN: llvm-dsymutil --version 2>&1 | FileCheck --check-prefix=VERSION %s
 VERSION: {{ version }}

llvm/trunk/tools/dsymutil/dsymutil.cpp

static opt<bool> DumpStab(
     init(false), cat(DsymCategory));
 static alias DumpStabA("s", desc("Alias for --symtab"), aliasopt(DumpStab));
 
 static opt<bool> FlatOut("flat",
                          desc("Produce a flat dSYM file (not a bundle)."),
                          init(false), cat(DsymCategory));
 static alias FlatOutA("f", desc("Alias for --flat"), aliasopt(FlatOut));
 
-static opt<unsigned> Threads(
-    "threads",
+static opt<unsigned> NumThreads(
+    "num-threads",
     desc("Specifies the maximum number (n) of simultaneous threads to use\n"
          "when linking multiple architectures."),
     value_desc("n"), init(0), cat(DsymCategory));
-static alias ThreadsA("t", desc("Alias for --threads"), aliasopt(Threads));
+static alias NumThreadsA("j", desc("Alias for --num-threads"),
+                         aliasopt(NumThreads));
 
 static opt<bool> Verbose("verbose", desc("Verbosity level"), init(false),
                          cat(DsymCategory));
 
 static opt<bool>
     NoOutput("no-output",
              desc("Do the link in memory, but do not emit the result file."),
              init(false), cat(DsymCategory));
if (auto EC = DebugMapPtrsOrErr.getError()) {
       exitDsymutil(1);
     }
 
     if (DebugMapPtrsOrErr->empty()) {
       llvm::errs() << "error: no architecture to link\n";
       exitDsymutil(1);
     }
 
-    unsigned NumThreads = Threads;
-    if (!NumThreads)
+    if (NumThreads == 0)
       NumThreads = llvm::thread::hardware_concurrency();
     if (DumpDebugMap || Verbose)
       NumThreads = 1;
-    NumThreads = std::min(NumThreads, (unsigned)DebugMapPtrsOrErr->size());
+    NumThreads = std::min<unsigned>(NumThreads, DebugMapPtrsOrErr->size());
 
     llvm::ThreadPool Threads(NumThreads);
 
     // If there is more than one link to execute, we need to generate
     // temporary files.
     bool NeedsTempFiles = !DumpDebugMap && (*DebugMapPtrsOrErr).size() != 1;
     llvm::SmallVector<MachOUtils::ArchAndFilename, 4> TempFiles;
     for (auto &Map : *DebugMapPtrsOrErr) {