Tighten up compiler flags for aidl

Turn all warnings to errors, with special cases for places where
yacc forces us to be lenient.  Remove one variable with 0 references
elsewhere in the code.

These flags are confined to be clang only because the Windows SDK build
is using an older compiler that does not understand these flags.

Change-Id: I6407bd331c25b8845bde46e9a917bce1d6857bd8