Suppress false-positive GCC -Wreturn-type warning.

llvm-svn: 361094
diff --git a/lld/ELF/Symbols.h b/lld/ELF/Symbols.h
index 921ffd7..9e283ce 100644
--- a/lld/ELF/Symbols.h
+++ b/lld/ELF/Symbols.h
@@ -444,6 +444,7 @@
   case Symbol::PlaceholderKind:
     return sizeof(Symbol);
   }
+  llvm_unreachable("unknown symbol kind");
 }
 
 inline void replaceSymbol(Symbol *Sym, const Symbol &New) {