fix various GCC 8.1.X function pointer warnings

[stooke]

Hi, and thanks for all the work on HarfBuzz.
I've been working to reduce warnings from the latest GCC, which is fairly strict.
This patch fixes some function parameter / template and other issues.
It'd be great if you consider accepting it.

This patch fixes these warnings:

....r/harfbuzz/hb-debug.hh:130:1: error: explicit specialization ‘void _hb_debug_msg_va(const char*, const void*, const char*, bool, unsigned int, int, const char*, __va_list_tag*) [with int max_level = 0]’ may be missing attributes [-Werror=missing-attributes]
_hb_debug_msg_va<0> (const char *what HB_UNUSED,

and:
.../harfbuzz/hb-set-private.hh:54:60: error: ‘void* memset(void*, int, size_t)’ writing to an object of type ‘hb_set_t::page_t::vector_t’ {aka ‘struct hb_vector_size_t<long long unsigned int, 64>’} with ‘private’ member ‘hb_vector_size_t<long long unsigned int, 64>::u’ [-Werror=class-memaccess]
inline void init1 (void) { memset (&v, 0xff, sizeof (v)); }

[behdad]

Not sure about the reinterpret_cast. Why did you need it? memset takes void*, and that should be automatic? Even then, why cast to char* instead of void* then? We typically just use C-style casts.

[stooke]

Thanks for merging my PR. You're absolutely right, while my cast works, casting to void* using a C-style cast will work too.

The warning is not that the type of the parameter is wrong, but that it's modifying a private member.
I use reinterpret_cast to show the code is doing something potentially quite dangerous, but I should have used void* instead of char* (the end result is the same, but yours is more correct).

I'm more than happy to submit a simple PR to change this to C-style cast to void*, and I'm sorry I didn't match your style to begin with - just let me know.

[ebraminio]

Yes. Feel free to do so :)

[behdad]

The warning is not that the type of the parameter is wrong, but that it's modifying a private member.

Ah, interesting. Maybe I rewrite the code to call a method instead. That makes sense.

[behdad]

Rewote the code. Please test.

[stooke]

Your new code seems to be fine, thanks.