[LLD] Simplify the demangleItanium function. NFC.

[mstorsjo]

Summary

Instead of returning an optional, just return the input string if demangling fails, as that's what all callers use anyway.

lld/Common/Strings.cpp

 #include "llvm/Support/GlobPattern.h"
 #include <algorithm>
 #include <mutex>
 #include <vector>
 
 using namespace llvm;
 using namespace lld;
 
-// Returns the demangled C++ symbol name for Name.
-Optional<std::string> lld::demangleItanium(StringRef name) {
+// Returns the demangled C++ symbol name for name.
+std::string lld::demangleItanium(StringRef name) {
   // itaniumDemangle can be used to demangle strings other than symbol
   // names which do not necessarily start with "_Z". Name can be
   // either a C or C++ symbol. Don't call demangle if the name
   // does not look like a C++ symbol name to avoid getting unexpected
   // result for a C symbol that happens to match a mangled type name.
   if (!name.startswith("_Z"))
-    return None;
+    return name;
 
-  std::string demangled = demangle(name);
-  if (demangled == name)
-    return None;
-  return demangled;
+  return demangle(name);
 }
 
 StringMatcher::StringMatcher(ArrayRef<StringRef> pat) {
   for (StringRef s : pat) {
     Expected<GlobPattern> pat = GlobPattern::create(s);
     if (!pat)
       error(toString(pat.takeError()));
     else

lld/ELF/SymbolTable.cpp

 // So, if "extern C++" feature is used, we need to demangle all known
 // symbols.
 StringMap<std::vector<Symbol *>> &SymbolTable::getDemangledSyms() {
   if (!demangledSyms) {
     demangledSyms.emplace();
     for (Symbol *sym : symVector) {
       if (!sym->isDefined() && !sym->isCommon())
         continue;
-      if (Optional<std::string> s = demangleItanium(sym->getName()))
-        (*demangledSyms)[*s].push_back(sym);
-      else
-        (*demangledSyms)[sym->getName()].push_back(sym);
+      (*demangledSyms)[demangleItanium(sym->getName())].push_back(sym);
     }
   }
   return *demangledSyms;
 }
 
 std::vector<Symbol *> SymbolTable::findByVersion(SymbolVersion ver) {
   if (ver.isExternCpp)
     return getDemangledSyms().lookup(ver.name);

lld/ELF/Symbols.cpp

 Defined *ElfSym::relaIpltStart;
 Defined *ElfSym::relaIpltEnd;
 Defined *ElfSym::riscvGlobalPointer;
 Defined *ElfSym::tlsModuleBase;
 
 // Returns a symbol for an error message.
 static std::string demangle(StringRef symName) {
   if (config->demangle)
-    if (Optional<std::string> s = demangleItanium(symName))
-      return *s;
+    return demangleItanium(symName);
   return symName;
 }
 namespace lld {
 std::string toString(const Symbol &b) { return demangle(b.getName()); }
 std::string toELFString(const Archive::Symbol &b) {
   return demangle(b.getName());
 }
 } // namespace lld

lld/include/lld/Common/Strings.h

 #include "llvm/ADT/Optional.h"
 #include "llvm/ADT/StringRef.h"
 #include "llvm/Support/GlobPattern.h"
 #include <string>
 #include <vector>
 
 namespace lld {
 // Returns a demangled C++ symbol name. If Name is not a mangled
-// name, it returns Optional::None.
-llvm::Optional<std::string> demangleItanium(llvm::StringRef name);
+// name, it returns name.
+std::string demangleItanium(llvm::StringRef name);
 
 std::vector<uint8_t> parseHex(llvm::StringRef s);
 bool isValidCIdentifier(llvm::StringRef s);
 
 // Write the contents of the a buffer to a file
 void saveBuffer(llvm::StringRef buffer, const llvm::Twine &path);
 
 // This class represents multiple glob patterns.

lld/wasm/Symbols.cpp

 void LazySymbol::fetch() { cast<ArchiveFile>(file)->addMember(&archiveSymbol); }
 
 std::string lld::toString(const wasm::Symbol &sym) {
   return lld::maybeDemangleSymbol(sym.getName());
 }
 
 std::string lld::maybeDemangleSymbol(StringRef name) {
   if (config->demangle)
-    if (Optional<std::string> s = demangleItanium(name))
-      return *s;
+    return demangleItanium(name);
   return name;
 }
 
 std::string lld::toString(wasm::Symbol::Kind kind) {
   switch (kind) {
   case wasm::Symbol::DefinedFunctionKind:
     return "DefinedFunction";
   case wasm::Symbol::DefinedDataKind: