[CMake] Bumps minimum version to 3.20.0. This partly undoes D137724. This change has been discussed on discourse https://discourse.llvm.org/t/rfc-upgrading-llvms-minimum-required-cmake-version/66193 Note this does not remove work-arounds for older CMake versions, that will be done in followup patches. Reviewed By: mehdi_amini, MaskRay, ChuanqiXu, to268, thieta, tschuett, phosek, #libunwind, #libc_vendors, #libc, #libc_abi, sivachandra, philnik, zibi Differential Revision: https://reviews.llvm.org/D144509 GitOrigin-RevId: 44c6b905f8527635e49bb3ea97dea315f92d38ec
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