Add -Werror.

Also, fixed several warnings (including one bug in generateCppImpl.cpp).

Test: compiles

Bug: 32559427

Change-Id: I78375b9052c0ad447ebac638258f25bbf562712f
diff --git a/hidl-gen_l.ll b/hidl-gen_l.ll
index 1275745..5dd3c85 100644
--- a/hidl-gen_l.ll
+++ b/hidl-gen_l.ll
@@ -71,6 +71,8 @@
 
 %option yylineno
 %option noyywrap
+%option nounput
+%option noinput
 %option reentrant
 %option bison-bridge
 %option bison-locations