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.

A new ErrorCode enumeration is present in PluginInterface which can
be used when returning an llvm::Error from offload and PluginInterface
functions.

This enum must be kept up to sync with liboffload's ol_errc_t enum, so
both are automatically generated from liboffload's enum definition.

Some error codes have also been shuffled around to allow for future
work. Note that this patch only adds the machinery; actual error codes
will be added in a future patch.

~~Depends on #137339 , please ignore first commit of this MR.~~ This has
been merged.