Clean up remaining executable_semantics clang-tidy warnings (#906)

Co-authored-by: Geoff Romer <gromer@google.com>

executable_semantics/syntax/bison_wrap.h

@@ -24,6 +24,7 @@ class BisonWrap {
   }
 
   // Support transparent conversion to the wrapped type.
+  // NOLINTNEXTLINE(google-explicit-constructor)
   operator T() { return Release(); }
 
   // Deliberately releases the contained value. Errors if not initialized.

executable_semantics/syntax/parse_and_lex_context.h

@@ -44,10 +44,10 @@ class ParseAndLexContext {
 }  // namespace Carbon
 
 // Gives flex the yylex prototype we want.
-#define YY_DECL                                                            \
-  Carbon::Parser::symbol_type yylex(Carbon::Nonnull<Carbon::Arena*> arena, \
-                                    yyscan_t yyscanner,                    \
-                                    Carbon::ParseAndLexContext& context)
+#define YY_DECL                                                         \
+  auto yylex(Carbon::Nonnull<Carbon::Arena*> arena, yyscan_t yyscanner, \
+             Carbon::ParseAndLexContext& context)                       \
+      -> Carbon::Parser::symbol_type
 
 // Declares yylex for the parser's sake.
 YY_DECL;