[lld-macho][nfc] lld/Common's `demangle()` is redundant for Mach-O

[int3]

Summary

The only thing that the Common implementation does is a check for
config->demangle, but D135189: [lld/mac] With -demangle, strip leading _ from non-mangled names added that check to
maybeDemangleSymbol, so there's no need to go through Common...

[oontvoo]

lld/MachO/Symbols.cpp

35

nit: demangle(symName.str().c_str()); ? (no need to make a new std::string?)

[int3]

lld/MachO/Symbols.cpp

35

demangle takes an std::string, so this would construct two std::strings :)

I can change it to .str() though

lld/MachO/Symbols.cpp

 //===- Symbols.cpp --------------------------------------------------------===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 
 #include "Symbols.h"
 #include "InputFiles.h"
 #include "SyntheticSections.h"
-#include "lld/Common/Strings.h"
+#include "llvm/Demangle/Demangle.h"
 
 using namespace llvm;
 using namespace lld;
 using namespace lld::macho;
 
 static_assert(sizeof(void *) != 8 || sizeof(Symbol) == 56,
               "Try to minimize Symbol's size; we create many instances");
 
 // The Microsoft ABI doesn't support using parent class tail padding for child
 // members, hence the _MSC_VER check.
 #if !defined(_MSC_VER)
 static_assert(sizeof(void *) != 8 || sizeof(Defined) == 88,
               "Try to minimize Defined's size; we create many instances");
 #endif
 
 static_assert(sizeof(SymbolUnion) == sizeof(Defined),
               "Defined should be the largest Symbol kind");
 
 // Returns a symbol name for an error message.
 static std::string maybeDemangleSymbol(StringRef symName) {
   if (config->demangle) {
     symName.consume_front("_");
-    return demangle(symName, true);
+    return demangle(symName.str());
   }
-  return std::string(symName);
+  return symName.str();
 }
 
 std::string lld::toString(const Symbol &sym) {
   return maybeDemangleSymbol(sym.getName());
 }
 
 std::string lld::toMachOString(const object::Archive::Symbol &b) {
   return maybeDemangleSymbol(b.getName());