- split test into two revisions
- clarify comments

Co-authored-by: Ralf Jung <post@ralfj.de>

Automatic Rustup

TB: Reserved + Protected + IM + lazy is a horrible combination that should not exist

As discovered by `@JoJoDeveloping,` the result of having both Protector exceptions on lazy locations (protectors only protect initialized bytes) and interior mutability exceptions for protected tags (Reserved IM does not accept foreign writes when protected) leads to some very undesirable results, namely that we cannot do spurious writes even on protected activated locations.

We propose that Protected Reserved IM should no longer exist and instead when a type is retagged as part of a `FnEntry` it is assumed to lose interior mutability.

In fact, this was already being done implicitly because relevant transitions were guarded by an `if protected`, but the difference is that now it also applies to transitions that occur after the end of the protector.

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Add `O_NOFOLLOW` flag support

Add `pread` and `pwrite` shims

Requested in rust-lang#2057.

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In order to support gettid when isolation is enabled and when it is
disabled, and satisfy its requirement that:

   In a single-threaded process, the thread ID is equal to the process ID
   (PID, as returned by getpid(2)).

we define the thread ID to be getpid() + <miri internal thread id>.
Since the internal thread id of the main thread is zero, this will
satisfy that requirement.

However, getpid for now was only supported when isolation was disabled.
To support getpid in isolation mode, we return a hardcoded value (1000)
and return that instead of the real PID.

Add `gettid` support

Add support for `gettid` in miri.

To ensure that the requirement that  `getpid() == gettdi()` for the main thread, we use the value returned by `getpid` and add to it the internal thread index.

Since `getpid` is only supported when isolation is disabled, and we want `gettid` to be used both in isolated and non-isolated executions, we modify `getpid` to return a hardcoded value (1000) when running in isolation mode.

Fixes rust-lang#3730

show warning when Stacked Borrows skips a reborrow due to 'extern type'

When this happens, we can't actually be sure to catch all bugs -- LLVM will still get a `noalias` but Miri can't do reborrowing. That's not good.

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better diagnostics for Tree Borrows + int2ptr casts

- Entirely reject `-Zmiri-permissive-provenance -Zmiri-tree-borrows` since that combination just doesn't work
- In the int2ptr cast warning, when Tree Borrows is enabled, do not recommend `-Zmiri-permissive-provenance`, instead note that Tree Borrows does not support int2ptr casts

Fixes rust-lang/miri#3764

Insert FileDescription instead of FileDescriptor in ``insert_fd``

This PR moves the creation of ``FileDescriptor`` inside ``insert_fd``, and ``insert_fd`` now takes in ``FileDescription`` instead of ``FileDescriptor``.  This change is needed by rust-lang#3712.

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