*mut T should coerce to *const T

[petrochenkov]

Currently only the last line in the example below doesn't compile.

fn fref(_: &u8) {}
fn fptr(_: *const u8) {}

let mut v = 0u8;
fref(&v);
fref(&mut v);
fptr(&v as *const u8);
fptr(&mut v as *mut u8); // error: mismatched types: expected `*const u8`, found `*mut u8` (values differ in mutability)

The conversion *mut T -> *const T is safe and consistent both with Rust references (&mut T coerces to &T) and C pointers (T* is implicitly convertible to const T*). I don't see the reasons why it is not allowed.

[petrochenkov]

Resolved by #19766