A version of `olCreateProgram` that inputs many bitcode files and links
them together before loading them.

The device handler for JIT processing images now accepts a list of
buffers rather than just one. Devices are expected to either link them
all into a single binary, or return an error code if they don't support
linking.

Currently, only amdgpu supports multiple binaries.

The `unloadBinaryImpl` method on the host plugin is now implemented
properly (rather than just being a stub). When an image is unloaded,
it is deallocated and the library associated with it is closed.

GenericKernelTy has a pointer to the name that was used to create it.
However, the name passed in as an argument may not outlive the kernel.
Instead, GenericKernelTy now contains a std::string, and copies the
name into there.

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.

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.

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`.

Previously we decided to check in files that we generate with tablegen.
The justification at the time was that it helped reviewers unfamiliar
with `offload-tblgen` see the actual changes to the headers in PRs.
After trying it for a while, it's ended up causing some headaches and is
also not how tablegen is used elsewhere in LLVM.

This changes our use of tablegen to be more conventional. Where
possible, files are still clang-formatted, but this is no longer a hard
requirement. Because `OffloadErrcodes.inc` is shared with libomptarget
it now gets generated in a more appropriate place.

[Offload] Use new error code handling mechanism

This removes the old ErrorCode-less error method and requires
every user to provide a concrete error code. All calls have been
updated.

In addition, for consistency with error messages elsewhere in LLVM, all
messages have been made to start lower case.