[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.

Summary:
This patch moves the RPC server handling to be a header only utility
stored in the `shared/` directory. This is intended to be shared within
LLVM for the loaders and `offload/` handling.

Generally, this makes it easier to share code without weird
cross-project binaries being plucked out of the build system. It also
allows us to soon move the loader interface out of the `libc` project so
that we don't need to bootstrap those and can build them in LLVM.

Summary:
If the user deallocates an RPC device this can sometimes fail if the RPC
server is still running. This will happen if the modification happens
while the server is still checking it. This patch adds a mutex to guard
modifications to it.

Summary:
This patch just changes the interface to make starting the thread
multiple times permissable since it will only be done the first time.
Note that this does not refcount it or anything, so it's onto the user
to make sure that they don't shut down the thread before everyone is
done using it. That is the case today because the shutDown portion is
run by a single thread in the destructor phase.

Another question is if we should make this thread truly global state,
because currently it will be private to each plugin instance, so if you
have an AMD and NVIDIA image there will be two, similarly if you have
those inside of a shared library.

Summary:
Handling the RPC server requires running through list of jobs that the
device has requested to be done. Currently this is handled by the thread
that does the waiting for the kernel to finish. However, this is not
sound on NVIDIA architectures and only works for async launches in the
OpenMP model that uses helper threads.

However, we also don't want to have this thread doing work
unnnecessarily. For this reason we track the execution of kernels and
cause the thread to sleep via a condition variable (usually backed by
some kind of futex or other intelligent sleeping mechanism) so that the
thread will be idle while no kernels are running.

Somehow conflict with define in wingdi.h.

Fix build failures:

[ 52%] Building CXX object
projects/offload/plugins-nextgen/common/CMakeFiles/PluginCommon.dir/src/RPC.cpp.obj
In file included from
...llvm\offload\plugins-nextgen\common\src\RPC.cpp:16:
...\llvm\libc\shared\rpc.h(48,3): error: expected identifier
   48 |   ERROR = 0x1000,
      |   ^
c:\Program files (x86)\Windows
Kits\10\include\10.0.22000.0\um\wingdi.h(118,29): note: expanded from
macro 'ERROR'
  118 | #define ERROR               0
      |                             ^
...\llvm\offload\plugins-nextgen\common\src\RPC.cpp(75,17): error:
expected unqualified-id
   75 |     return rpc::ERROR;
      |                 ^
c:\Program files (x86)\Windows
Kits\10\include\10.0.22000.0\um\wingdi.h(118,29): note: expanded from
macro 'ERROR'
  118 | #define ERROR               0
      |                             ^
2 errors generated.

Summary:
RPC_ is a generic prefix here, use LIBC_ to indicate that these are
opcodes used to implement the C library

Summary:
This patch adds an RPC interface that lives directly in the OpenMP
device runtime. This allows OpenMP to implement custom opcodes.
Currently this is only providing the host call interface, which is the
raw version of reverse offloading. Previously this lived in `libc/` as
an extension which is not the correct place.

The interface here uses a weak symbol for the RPC client by the same
name that the `libc` interface uses. This means that it will defer to
the libc one if both are present so we don't need to set up multiple
instances.

The presense of this symbol is what controls whether or not we set up
the RPC server. Because this is an external symbol it normally won't be
optimized out, so there's a special pass in OpenMPOpt that deletes this
symbol if it is unused during linking. That means at `O0` the RPC server
will always be present now, but will be removed trivially if it's not
used at O1 and higher.

Summary:
We can simply include this header from the shared directory now and do
not need to have this level of indirection. Simply stash it with the
other libc opcode handlers.

If we were able to move the printf handlers to the shared directory then
this could just be a header as well, which would HEAVILY simplify the
mess associated with building the RPC server first in the projects
build, then copying it to the runtimes build.