Fix a bunch of crashes discovered by fuzzing cppfront

[threeifbyair]

Here are fixes for several cppfront crashes discovered from fuzzing.

(1) Errors found in metafunctions cause cppfront to end with a signal instead of exiting cleanly.
(2) Two #else operations cause an assertion instead of an error.
(3) $ is allowed in preconditions and assertions but crashes cppfront when it tries to emit cpp1.
(4) 'unsigned char' close to the end of the file causes an assertion.
(5) Comments near the end of the file cause an assertion.
(6) If the initializer of a parameter is not an expression, cppfront segfaults.
(7) If a type defines a function before stating a superclass, cppfront crashes. (Closes #1382)
(8) Some invalid aliases crash cppfront.
(9) 'decltype' in the wrong place crashes cppfront.

Of these, only (5) was found on legal C++2 code. The rest should have generated (and now do generate) errors.

[hsutter]

Thanks! This is a nice set of fixed, appreciated. Fuzzing FTW 🏅