Reuse LLVM build tree for MLIR python bindings

[mitchdz]

The wheel container previously re-ran build_llvm.sh per Python version with
python-bindings added, which reconfigured from scratch and recompiled the
full LLVM/MLIR/clang/lld tree (~5,800 objects, ~65 min).

The manylinux base already builds and retains /llvm-project/build without
bindings. Add a focused script that reuses that tree, flips
MLIR_ENABLE_BINDINGS_PYTHON on, and runs only the install-MLIRPythonModules
and install-mlir-python-sources targets. nanobind is still rebuilt per
Python since it ABI-binds to the interpreter.

[atgeller]

Nice. Thanks @mitchdz!

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 78.72%. Comparing base (7744cc7) to head (559c909).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4488      +/-   ##
==========================================
+ Coverage   78.71%   78.72%   +0.01%     
==========================================
  Files         534      534              
  Lines       69782    69782              
==========================================
+ Hits        54926    54935       +9     
+ Misses      14856    14847       -9     

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[github-actions]

CI Summary (push) — ✅ passed

Run #25583205299 · ✅ 6 · ⏩ 7 · ❌ 0 · ⛔ 0

Top-level jobs (13)

Job	Result
binaries	⏩ skipped
build_and_test	✅ success
config_devdeps	✅ success
config_source_build	⏩ skipped
config_wheeldeps	✅ success
devdeps	✅ success
docker_image	⏩ skipped
gen_code_coverage	⏩ skipped
metadata	✅ success
python_metapackages	⏩ skipped
python_wheels	⏩ skipped
source_build	⏩ skipped
wheeldeps	✅ success

⏩ Skipped jobs (7) — intentionally skipped on PR builds; run on merge_group / workflow_dispatch

Job
binaries
config_source_build
docker_image
gen_code_coverage
python_metapackages
python_wheels
source_build

All sub-jobs (42) — every matrix leg, with links

Job	Status	Link
Build and test (amd64, gcc12, openmpi) / Dev environment (Debug)	✅ success	view
Build and test (amd64, gcc12, openmpi) / Dev environment (Python)	✅ success	view
Build and test (amd64, llvm, openmpi) / Dev environment (Debug)	✅ success	view
Build and test (amd64, llvm, openmpi) / Dev environment (Python)	✅ success	view
Build and test (arm64, llvm, openmpi) / Dev environment (Debug)	✅ success	view
Build and test (arm64, llvm, openmpi) / Dev environment (Python)	✅ success	view
CI Summary	❔ in_progress	view
Configure build (devdeps)	✅ success	view
Configure build (source_build)	⏩ skipped	view
Configure build (wheeldeps)	✅ success	view
Create CUDA Quantum installer	⏩ skipped	view
Create Docker images	⏩ skipped	view
Create Python metapackages	⏩ skipped	view
Create Python wheels	⏩ skipped	view
Gen code coverage	⏩ skipped	view
Load dependencies (amd64, gcc12) / Caching	✅ success	view
Load dependencies (amd64, gcc12) / Finalize	✅ success	view
Load dependencies (amd64, gcc12) / Metadata	✅ success	view
Load dependencies (amd64, llvm) / Caching	✅ success	view
Load dependencies (amd64, llvm) / Finalize	✅ success	view
Load dependencies (amd64, llvm) / Metadata	✅ success	view
Load dependencies (arm64, gcc12) / Caching	✅ success	view
Load dependencies (arm64, gcc12) / Finalize	✅ success	view
Load dependencies (arm64, gcc12) / Metadata	✅ success	view
Load dependencies (arm64, llvm) / Caching	✅ success	view
Load dependencies (arm64, llvm) / Finalize	✅ success	view
Load dependencies (arm64, llvm) / Metadata	✅ success	view
Load source build cache	⏩ skipped	view
Load wheel dependencies (amd64, 12.6) / Caching	✅ success	view
Load wheel dependencies (amd64, 12.6) / Finalize	✅ success	view
Load wheel dependencies (amd64, 12.6) / Metadata	✅ success	view
Load wheel dependencies (amd64, 13.0) / Caching	✅ success	view
Load wheel dependencies (amd64, 13.0) / Finalize	✅ success	view
Load wheel dependencies (amd64, 13.0) / Metadata	✅ success	view
Load wheel dependencies (arm64, 12.6) / Caching	✅ success	view
Load wheel dependencies (arm64, 12.6) / Finalize	✅ success	view
Load wheel dependencies (arm64, 12.6) / Metadata	✅ success	view
Load wheel dependencies (arm64, 13.0) / Caching	✅ success	view
Load wheel dependencies (arm64, 13.0) / Finalize	✅ success	view
Load wheel dependencies (arm64, 13.0) / Metadata	✅ success	view
Prepare cache clean-up	✅ success	view
Retrieve PR info	✅ success	view

✅ Required checks (6/6) — declared in .github/required-checks.yml for push

Required check	Status	Link
Build and test (amd64, llvm, openmpi) / Dev environment (Debug)	✅ success	view
Build and test (amd64, llvm, openmpi) / Dev environment (Python)	✅ success	view
Build and test (arm64, llvm, openmpi) / Dev environment (Debug)	✅ success	view
Build and test (arm64, llvm, openmpi) / Dev environment (Python)	✅ success	view
Build and test (amd64, gcc12, openmpi) / Dev environment (Debug)	✅ success	view
Build and test (amd64, gcc12, openmpi) / Dev environment (Python)	✅ success	view