This reverts commit 84a214856ad989f37af19f5e8aaa9ec2346dde6f.

This gives us more time to work out the alternative and also people to
migrate

Inspired by this comment
https://github.com/llvm/llvm-project/pull/157930#issuecomment-3346634290
(and long-standing issues related to finding nanobind/pybind in the
right place), this PR moves to using `FetchContent_Declare` to get the
nanobind dependency. This is pretty standard (see e.g.,
[IREE](https://github.com/iree-org/iree/blob/cf60359b7443b0e92e15fb6ffc011525dc40e772/CMakeLists.txt#L842-L848)).
This PR also removes pybind which has been deprecated for almost a year
(https://github.com/llvm/llvm-project/pull/117922) and which isn't
compatible (for whatever reason) with `FetchContent_Declare`.

---------

Co-authored-by: Jacques Pienaar <jpienaar@google.com>

This a reland of https://github.com/llvm/llvm-project/pull/155741 which
was reverted at https://github.com/llvm/llvm-project/pull/157831. This
version is narrower in scope - it only turns on automatic stub
generation for `MLIRPythonExtension.Core._mlir` and **does not do
anything automatically**. Specifically, the only CMake code added to
`AddMLIRPython.cmake` is the `mlir_generate_type_stubs` function which
is then used only in a manual way. The API for
`mlir_generate_type_stubs` is:

```
Arguments:
  MODULE_NAME: The fully-qualified name of the extension module (used for importing in python).
  DEPENDS_TARGETS: List of targets these type stubs depend on being built; usually corresponding to the
    specific extension module (e.g., something like StandalonePythonModules.extension._standaloneDialectsNanobind.dso)
    and the core bindings extension module (e.g., something like StandalonePythonModules.extension._mlir.dso).
  OUTPUT_DIR: The root output directory to emit the type stubs into.
  OUTPUTS: List of expected outputs.
  DEPENDS_TARGET_SRC_DEPS: List of cpp sources for extension library (for generating a DEPFILE).
  IMPORT_PATHS: List of paths to add to PYTHONPATH for stubgen.
  PATTERN_FILE: (Optional) Pattern file (see https://nanobind.readthedocs.io/en/latest/typing.html#pattern-files).
Outputs:
  NB_STUBGEN_CUSTOM_TARGET: The target corresponding to generation which other targets can depend on.
```

Downstream users should use `mlir_generate_type_stubs` in coordination
with `declare_mlir_python_sources` to turn on stub generation for their
own downstream dialect extensions and upstream dialect extensions if
they so choose. Standalone example shows an example.

Note, downstream will also need to set
`-DMLIR_PYTHON_PACKAGE_PREFIX=...` correctly for their bindings.

This is mentioned as a "must" in
https://nanobind.readthedocs.io/en/latest/porting.html#type-casters when
implementing type casters.

While most of the existing `from_cpp` methods were already marked
noexcept, many of the `from_python` methods were not. This commit adds
the missing noexcept declarations to all type casters found in
`NanobindAdaptors.h`.

---------

Co-authored-by: Maksim Levental <maksim.levental@gmail.com>

Updated the Python diagnostics handler to emit notes (in addition to
errors) into the output stream so that users have more context as to
where in the IR the error is occurring.

Use `mlir_target_link_libraries()` to link dependencies of libraries
that are not included in libMLIR, to ensure that they link to the dylib
when they are used in Flang. Otherwise, they implicitly pull in all
their static dependencies, effectively causing Flang binaries to
simultaneously link to the dylib and to static libraries, which is never
a good idea.

I have only covered the libraries that are used by Flang. If you wish, I
can extend this approach to all non-libMLIR libraries in MLIR, making
MLIR itself also link to the dylib consistently.

[v3 with more `-DBUILD_SHARED_LIBS=ON` fixes]

This reverts commit 4c6242ebf50dde0597df2bace49d534b61122496.  More
BUILD_SHARED_LIBS=ON regressions, sigh.

Use `mlir_target_link_libraries()` to link dependencies of libraries
that are not included in libMLIR, to ensure that they link to the dylib
when they are used in Flang. Otherwise, they implicitly pull in all
their static dependencies, effectively causing Flang binaries to
simultaneously link to the dylib and to static libraries, which is never
a good idea.

I have only covered the libraries that are used by Flang. If you wish, I
can extend this approach to all non-libMLIR libraries in MLIR, making
MLIR itself also link to the dylib consistently.

[v2 with fixed `-DBUILD_SHARED_LIBS=ON` build]

This reverts commit af6616676fb7f9dd4898290ea684ee0c90f1701d.  It broke
builds with `-DBUILD_SHARED_LIBS=ON`.

Use `mlir_target_link_libraries()` to link dependencies of libraries
that are not included in libMLIR, to ensure that they link to the dylib
when they are used in Flang. Otherwise, they implicitly pull in all
their static dependencies, effectively causing Flang binaries to
simultaneously link to the dylib and to static libraries, which is never
a good idea.

I have only covered the libraries that are used by Flang. If you wish, I
can extend this approach to all non-libMLIR libraries in MLIR, making
MLIR itself also link to the dylib consistently.