Segfault when reassigning to a const slice parameter

[lukechampine]

The following test should result in a compile-time error, "cannot assign to constant":

pub fn reassign(s: []const u8) void {
    s = s[0..];
}

test "reassign slice" {
    const foo = "foo";
    reassign(foo);
}

Instead, it segfaults on Windows and Linux, and passes (erroneously) on MacOS.

If foo is declared as a var, or if reassign takes a []u8, the test passes (erroneously) instead of segfaulting. Presumably this is because the string is no longer being stored in read-only memory. If the reassignment is changed to s = s, the compiler correctly emits an error. So my guess is that the slice expression is tripping up the "assign to constant" check.

[ghost]

Maybe related #3800