Add underscores to private data members (#898)

This is the last PR I plan to have focused on #720

executable_semantics/syntax/bison_wrap.h

@@ -19,7 +19,7 @@ class BisonWrap {
  public:
   // Assigning a value initializes the wrapper.
   auto operator=(T&& rhs) -> BisonWrap& {
-    val = std::move(rhs);
+    val_ = std::move(rhs);
     return *this;
   }
 
@@ -29,14 +29,14 @@ class BisonWrap {
   // Deliberately releases the contained value. Errors if not initialized.
   // Called directly in parser.ypp when releasing pairs.
   auto Release() -> T {
-    CHECK(val.has_value());
-    T ret = std::move(*val);
-    val.reset();
+    CHECK(val_.has_value());
+    T ret = std::move(*val_);
+    val_.reset();
     return ret;
   }
 
  private:
-  std::optional<T> val;
+  std::optional<T> val_;
 };
 
 }  // namespace Carbon