[WebAssembly] Avoid COMDAT hashmap lookup for each symbol. NFC

This reduces the number of lookups to one per COMDAT group, rather than
one per symbol in a COMDAT group.

Differential Revision: https://reviews.llvm.org/D44344

llvm-svn: 327523
diff --git a/lld/wasm/SymbolTable.cpp b/lld/wasm/SymbolTable.cpp
index a722472..153b507 100644
--- a/lld/wasm/SymbolTable.cpp
+++ b/lld/wasm/SymbolTable.cpp
@@ -304,6 +304,6 @@
   }
 }
 
-bool SymbolTable::addComdat(StringRef Name, const ObjFile *File) {
-  return Comdats.insert({Name, File}).first->second == File;
+bool SymbolTable::addComdat(StringRef Name) {
+  return Comdats.insert(CachedHashStringRef(Name)).second;
 }