[llvm-objdump] Use std::make_unique

Fix up my recent commit rG1128311a19179ceca799ff0fbc4dd206ab56e560 to
use std::make_unique instead of std::unique_ptr(new), as requested by
David Blaikie.

Differential Revision: https://reviews.llvm.org/D101822

llvm/tools/llvm-objdump/llvm-objdump.cpp

@@ -1015,7 +1015,7 @@ static void addSymbolizer(
                     LabelAddrs.begin());
   // Add the labels.
   for (unsigned LabelNum = 0; LabelNum != LabelAddrs.size(); ++LabelNum) {
-    std::unique_ptr<std::string> Name(new std::string);
+    auto Name = std::make_unique<std::string>();
     *Name = (Twine("L") + Twine(LabelNum)).str();
     SynthesizedLabelNames.push_back(std::move(Name));
     Symbols.push_back(SymbolInfoTy(