Refactor Type::toCBuffer* and helpers into a visitor class in hdrgen.c

- Move the main function Type::toCBuffer2 into the visitor class
- Move TypeQualified::toCBuffer2Helper into an internal helper function
- Remove one-caller function attributesToCBuffer into caller
- Factor out repeated safety and linkage printing
- Rename functionToCBuffer2 to functionToBufferWithIdent
- Rename toCBufferWithAttributes to functionToBufferFull
- Do special-case checking in Type::toCBuffer instead of overriding in TypeFunction::toCBuffer and TypeError::toCBuffer
- Move Type::toCBuffer3 into internal helper visitWithMask
- Rename toCBuffer2 to toBufferShort and make it a free function
- Move Type::toCBuffer into a free function