Add an option to disable use of bundled LLVM. (#12256)

* Adds an option `IREE_BUILD_BUNDLED_LLVM` (default ON) which builds
LLVM+tools from the in-repo submodule (`third_party/llvm-project`). When
disabled, CMake must be configured such that find_package for `LLVM`,
`MLIR`, and `LLD` functions and are at commit levels appropriate.
* Re-organizes the LLVM cmake setup to be better contained and
normalized vs in-tree/bundled and installed.
* Switches MLIR sub-projects iree-dialects, mlir-hlo and torch-dialects
to configure from the main IREE build so that they can be used on top of
a locally built or installed LLVM.
* Updates mechanisms for locating lit so that it is consistent between
the two modes.
* Moves the binary directory for `third_party/llvm-project/llvm` ->
`llvm-project`. This means that folks interacting with the bundled LLVM
in the build directory will say things like `ninja llvm-project/all` or
`./llvm-project/bin/mlir-opt`.
* Moves the MLIR sub-projects from under
`llvm-project/tools/{subproject}` ->
`llvm-external-projects/{subproject}` so that it is consistent between
bundled and installed modes. Practically, this means that if invoking
`iree-dialects-opt` from the command line, it is in a different place.
* Fix to a transform dialect ODR issue which was latent and a namespace
conflict with upstream. Not sure what triggered it but put it down to
the vagaries of linking.
* Fix to the VMVX dependent dialects that came up when building IREE
with all backends disabled.

With this, I was able to install my own LLVM with these commands:

```
cd third_party/llvm-project
mkdir build
cd build
cmake -G Ninja ../llvm \
   -DLLVM_ENABLE_PROJECTS="mlir;lld" \
   -DLLVM_BUILD_EXAMPLES=OFF \
   -DLLVM_TARGETS_TO_BUILD="" \
   -DLLVM_INSTALL_UTILS=ON \
   -DCMAKE_BUILD_TYPE=Release \
   -DLLVM_ENABLE_ASSERTIONS=ON \
   -DCMAKE_C_COMPILER=clang -DCMAKE_CXX_COMPILER=clang++ -DLLVM_ENABLE_LLD=ON \
   -DLLVM_CCACHE_BUILD=ON
ninja
cmake -DCMAKE_INSTALL_PREFIX=$HOME/tmp/llvm -P cmake_install.cmake
```

```
cmake -GNinja -B ../iree-build-inst/ -S . \
    -DIREE_TARGET_BACKEND_DEFAULTS=OFF \
    -DIREE_HAL_DRIVER_DEFAULTS=OFF \
    -DIREE_HAL_DRIVER_LOCAL_SYNC=ON \
    -DIREE_HAL_DRIVER_LOCAL_TASK=ON \
    -DIREE_BUILD_BUNDLED_LLVM=OFF \
    -DLLVM_DIR=$HOME/tmp/llvm/lib/cmake/llvm \
    -DMLIR_DIR=$HOME/tmp/llvm/lib/cmake/mlir \
    -DLLD_DIR=$HOME/tmp/llvm/lib/cmake/lld \
    -DCMAKE_BUILD_TYPE=Release
```

* Python builds have not yet been tested in the installed path and need
some ergonomic help.
* Will follow-up with a script to build various configurations of
LLVM/MLIR/LLD so that we can wire it up to CI.

Progress on #12231