Bump minimum toolchain version

RFC: https://discourse.llvm.org/t/rfc-increasing-the-gcc-and-clang-requirements-to-support-c-17-in-llvm

Following the policy here: https://llvm.org/docs/DeveloperPolicy.html#toolchain

This forum post here will be updated with the timeline and status: https://discourse.llvm.org/t/important-new-toolchain-requirements-to-build-llvm-will-most-likely-be-landing-within-a-week-prepare-your-buildbots/61447

Reviewed By: mehdi_amini, jyknight, jhenderson, cor3ntin, MaskRay

Differential Revision: https://reviews.llvm.org/D122976