Fix <unimplemented> for size_t return type on builtins

[davispuh]

Currently if you have code like this:

#include  <stdio.h>

size_t strlen(const char *__s);
size_t fread();

It's translated to:

extern (C):

<unimplemented> strlen (const(char)* __s);
<unimplemented> fread ();

Which obviously is invalid code.
This PR fixes this to create correct D code.

Note that this issue happens only with compiler's builtin functions that return size_t because clang merges it to internal unexposed type.

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