Bump LLVM to minimum >= 12 #4354

Job	Run time
VFX2021 gcc9/C++17 llvm11 py3.7 oiio2.5 sse4 batch-b8avx2	3m 12s
VFX2023 gcc11/C++17 llvm15 py3.10 oiio-rel avx2	12m 59s
VFX2022 gcc9/C++17 llvm13 py3.9 exr3.1 oiio-rel avx2	12m 58s
macos-13 C++17 py3.11 MacOS-13 llvm17	13m 12s
VFX2023 GPU Cuda11 gcc11/C++17 llvm15 py3.10 OIIO-main avx2	6m 27s
VFX2022 clang12/C++17 llvm12 oiio-main py3.9 avx2 batch-avx512	13m 0s
VFX2024 gcc11/C++17 llvm17 py3.11 boost1.82 oiio-rel avx2	13m 1s
Ubuntu Debug gcc7/C++17 llvm12 py3.8 oiio2.5 exr3.1 sse4	1m 5s
VFX2022 gcc9/C++17 llvm11 py3.9 exr3.1 oiio3.0 sse2 batch-b4sse2	13m 1s
Ubuntu gcc10/C++17 llvm12 oiio-2.5 avx2	43s
VFX2024 sanitizers	12m 59s
VFX2021 oldest everything gcc9/C++17 llvm9 py3.7 oiio2.5 no-simd	3m 13s
Ubuntu latest releases gcc11/C++17 llvm17 oiio-3.0 exr3.2 py3.12 avx2 batch-b16avx512	13m 11s
VFX2023 abi check	11m 19s
macos-14 C++17 py3.11 MacOS-14-ARM llvm17	8m 54s
Ubuntu bleeding edge gcc12/C++17 llvm17 oiio/ocio/exr/pybind-main py3.12 avx2 batch-b16avx512	12m 57s
VFX2023 icc/C++17 llvm14 py3.9 oiio-main avx2	13m 1s
Ubuntu clang14/C++17 llvm14 py3.8 avx2 batch-b16avx512	12m 57s
VFX2023 icx/C++17 llvm14 py3.10 oiio-3.0 avx2	13m 0s
Ubuntu clang-format	4m 59s
	3h 16m 8s

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