Add a block for C clients in the compatibility header. This block
contains only the `@cdecl` functions that are printed using only C
types.
This C block is printed above the Objective-C and C++ blocks as if we
add support for `@cdecl` types other languages should be able to
reference them in function signatures. Other languages block don't
duplicate printing the `@cdecl` functions either as they are already
accessible to them.
