dmd.saferPrintf.dd

Allow certain `pragma(printf)` function calls to be treated as `@safe`

`printf` function calls in general are unsafe. Many calls, however,
can be automatically checked for safety.

This change allows calling a $(DDSUBLINK spec/pragma, printf,
`pragma(printf)`) function from @safe code when:

- the callee is marked `@safe` or `@trusted`
- the format string passed is a literal
- no string format specifiers are used
- the callee has a `...` parameter after the format string

Note: `pragma(printf)` already enforces type safety for a C-style
variadic function.

For example:

```d
extern(C) pragma(printf)
void printf(const char* format, ...) @trusted;

@safe void func(int i, char* s)
{
    printf("i is %d\n", i);  // allowed
    printf("s is %s\n", s);  // Error: call is not safe
    printf(s);               // Error: call is not safe
}
```