Change function type more LLVM-ish.

Also fix variable naming style.

llvm-svn: 320393
diff --git a/lld/wasm/Driver.cpp b/lld/wasm/Driver.cpp
index a0aec17..983d8e9 100644
--- a/lld/wasm/Driver.cpp
+++ b/lld/wasm/Driver.cpp
@@ -202,10 +202,10 @@
     error("no input files");
 }
 
-static const char *getEntry(opt::InputArgList &Args, const char *def) {
+static StringRef getEntry(opt::InputArgList &Args, StringRef Default) {
   auto *Arg = Args.getLastArg(OPT_entry, OPT_no_entry);
   if (!Arg)
-    return def;
+    return Default;
   if (Arg->getOption().getID() == OPT_no_entry)
     return "";
   return Arg->getValue();