Silence some noisy, benign warnings

[sampsyo]

Two kinds of warnings appear when compiling correct EnerC code:

• Cast errors saying stuff like "casting int * to parameter of type Q1 int *" are a result of our adapting the types of functions before getting to adapt the type of the function call. Maybe we can trick Clang into checking these as the same type.
• Endorsements sometimes cause an "unused value" warning, despite our attempts to turn them off with pragmas.

These may be solved automatically if we ever switch to Quala for our frontend needs.