[Runtime] Make hash_value function inline.

Author: DougGregor

stdlib/public/runtime/MetadataLookup.cpp

@@ -196,8 +196,10 @@ namespace {
   };
 } // end anonymous namespace
 
-llvm::hash_code llvm::hash_value(StringRef S) {
-  return hash_combine_range(S.begin(), S.end());
+namespace {
+  inline llvm::hash_code llvm::hash_value(StringRef S) {
+    return hash_combine_range(S.begin(), S.end());
+  }
 }
 
 struct TypeMetadataPrivateState {