This adds a new device info query for the maximum workgroup/block size
for each dimension.

This makes several small changes to how the platform and device info
queries are handled:
* ReturnHelper has been replaced with InfoWriter which is more explicit
  in how it is invoked.
* InfoWriter consumes `llvm::Expected` rather than values directly, and
  will early exit if it returns an error.
* As a result of the above, `GetInfoString` now correctly returns errors
  rather than empty strings.
* The host device now has its own dedicated "getInfo" function rather
  than being checked in multiple places.

`olShutDown` was not properly calling deinit on the platforms, resulting
in random segfaults on AMD devices.

As part of this, `olInit` and `olShutDown` now alloc and free the
offload context rather than it being static. This
allows `olShutDown` to be called within a destructor of a static object
(like the tests do) without having to worry about destructor ordering.

Rather than creating a new device info tree for each call to
`olGetDeviceInfo`, we instead do it on device initialisation. As well
as improving performance, this fixes a few lifetime issues with returned
strings.

This does unfortunately mean that device information is immutable,
but hopefully that shouldn't be a problem for any queries we want to
implement.

This also meant allowing offload initialization to fail, which it can
now do.

This allows removal of a specific Image from a Device, rather than
requiring all image data to outlive the device they were created for.

This is required for `ol_program_handle_t`s, which now specify the
lifetime of the buffer used to create the program.

Previously, if a binary failed to load due to failures when jit
compiling, the function would return success with nullptr. Now it
returns a new plugin error, `COMPILE_FAILURE`.

Rather than being "stringly typed", store values as a std::variant that
can hold various types. This means that liboffload doesn't have to do
any string parsing for integer/bool device info keys.

All entry points (except olInit) now check that offload has been
initialized. If not, a new `OL_ERRC_UNINITIALIZED` error is returned.

Rather than having a number of static local variables, we now use
a single `OffloadContext` struct to store global state. This is
initialised by `olInit`, but is never deleted (de-initialization of
Offload isn't yet implemented).

The error reporting mechanism has not been moved to the struct, since
that's going to cause issues with teardown (error messages must outlive
liboffload).

Previously, device info was returned as a queue with each element having
a "Level" field indicating its nesting level. This replaces this queue
with a more traditional tree-like structure.

This should not result in a change to the output of
`llvm-offload-device-info`.