[llvm] Fix minimum Apple Clang requirement

This was stated as 9.3, but as pointed out in
https://discourse.llvm.org/t/rfc-bump-minimal-requirements-apple-clang-9-3-10-0-0-before-4th-tue-in-january/66156/7?u=kadircet
9.3 doesn't exist, hence this was effectively 10.0.

This patch merely reflects the reality more closely.

Differential Revision: https://reviews.llvm.org/D136609