Support generating (simple) macros in header-translator

[madsmtm]

Swift has support for this. From Apple's documentation:

simple, constant-like macros, declared with the #define directive

Macros are imported when they use literals for string, floating-point, or integer values, or use operators like +, -, >, and == between literals or previously defined macros.

Might be difficult if the macros' data isn't exposed properly to us by libclang (so hard to tell if the macro is sufficiently simple).

This is needed for better CoreFoundation support: #556.

[madsmtm]

Oops, I already had an issue open for this: #609