Fix build breakage.

llvm-svn: 305726

lld/COFF/Strings.cpp

@@ -24,7 +24,7 @@ Optional<std::string> coff::demangle(StringRef S) {
 #if defined(_MSC_VER)
   // UnDecorateSymbolName is not thread-safe, so we need a mutex.
   static std::mutex Mu;
-  std::lock_guard<std::mutex> Lock(mu);
+  std::lock_guard<std::mutex> Lock(Mu);
 
   char Buf[4096];
   if (S.startswith("?"))