[Reland][Libomptarget] Statically link all plugin runtimes (#87009)

This patch overhauls the `libomptarget` and plugin interface. Currently,
we define a C API and compile each plugin as a separate s

[Reland][Libomptarget] Statically link all plugin runtimes (#87009)

This patch overhauls the `libomptarget` and plugin interface. Currently,
we define a C API and compile each plugin as a separate shared library.
Then, `libomptarget` loads these API functions and forwards its internal
calls to them. This was originally designed to allow multiple
implementations of a library to be live. However, since then no one has
used this functionality and it prevents us from using much nicer
interfaces. If the old behavior is desired it should instead be
implemented as a separate plugin.

This patch replaces the `PluginAdaptorTy` interface with the
`GenericPluginTy` that is used by the plugins. Each plugin exports a
`createPlugin_<name>` function that is used to get the specific
implementation. This code is now shared with `libomptarget`.

There are some notable improvements to this.
1. Massively improved lifetimes of life runtime objects
2. The plugins can use a C++ interface
3. Global state does not need to be duplicated for each plugin +
libomptarget
4. Easier to use and add features and improve error handling
5. Less function call overhead / Improved LTO performance.

Additional changes in this plugin are related to contending with the
fact that state is now shared. Initialization and deinitialization is
now handled correctly and in phase with the underlying runtime, allowing
us to actually know when something is getting deallocated.

Depends on https://github.com/llvm/llvm-project/pull/86971
https://github.com/llvm/llvm-project/pull/86875
https://github.com/llvm/llvm-project/pull/86868

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Revert "[Libomptarget] Statically link all plugin runtimes (#87009)"

Caused failures on build-bots, reverting to investigate.

This reverts commit 80f9e814ec896fdc57ee84afad8ac4cb1f8e4627.

[Libomptarget] Statically link all plugin runtimes (#87009)

This patch overhauls the `libomptarget` and plugin interface. Currently,
we define a C API and compile each plugin as a separate shared li

[Libomptarget] Statically link all plugin runtimes (#87009)

This patch overhauls the `libomptarget` and plugin interface. Currently,
we define a C API and compile each plugin as a separate shared library.
Then, `libomptarget` loads these API functions and forwards its internal
calls to them. This was originally designed to allow multiple
implementations of a library to be live. However, since then no one has
used this functionality and it prevents us from using much nicer
interfaces. If the old behavior is desired it should instead be
implemented as a separate plugin.

This patch replaces the `PluginAdaptorTy` interface with the
`GenericPluginTy` that is used by the plugins. Each plugin exports a
`createPlugin_<name>` function that is used to get the specific
implementation. This code is now shared with `libomptarget`.

There are some notable improvements to this.
1. Massively improved lifetimes of life runtime objects
2. The plugins can use a C++ interface
3. Global state does not need to be duplicated for each plugin +
libomptarget
4. Easier to use and add features and improve error handling
5. Less function call overhead / Improved LTO performance.

Additional changes in this plugin are related to contending with the
fact that state is now shared. Initialization and deinitialization is
now handled correctly and in phase with the underlying runtime, allowing
us to actually know when something is getting deallocated.

Depends on https://github.com/llvm/llvm-project/pull/86971
https://github.com/llvm/llvm-project/pull/86875
https://github.com/llvm/llvm-project/pull/86868

show more ...

[Offload] Move `/openmp/libomptarget` to `/offload` (#75125)

In a nutshell, this moves our libomptarget code to populate the offload
subproject.

With this commit, users need to enable the new LL

[Offload] Move `/openmp/libomptarget` to `/offload` (#75125)

In a nutshell, this moves our libomptarget code to populate the offload
subproject.

With this commit, users need to enable the new LLVM/Offload subproject
as a runtime in their cmake configuration.
No further changes are expected for downstream code.

Tests and other components still depend on OpenMP and have also not been
renamed. The results below are for a build in which OpenMP and Offload
are enabled runtimes. In addition to the pure `git mv`, we needed to
adjust some CMake files. Nothing is intended to change semantics.

```
ninja check-offload
```
Works with the X86 and AMDGPU offload tests

```
ninja check-openmp
```
Still works but doesn't build offload tests anymore.

```
ls install/lib
```
Shows all expected libraries, incl.
- `libomptarget.devicertl.a`
- `libomptarget-nvptx-sm_90.bc`
- `libomptarget.rtl.amdgpu.so` -> `libomptarget.rtl.amdgpu.so.18git`
- `libomptarget.so` -> `libomptarget.so.18git`

Fixes: https://github.com/llvm/llvm-project/issues/75124

---------

Co-authored-by: Saiyedul Islam <Saiyedul.Islam@amd.com>

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