- Support macOS 11+ version scheme
- Standardize substitution name %min_deployment_target=x.y
- Remove unneeded error cases (the input version is hard-coded)
- Specify version as tuple instead of string; no need to parse it
These changes should also facilitate a future addition of a substitution
that expands to "set deployment target to current target version"
(https://reviews.llvm.org/D70151).
clang-format not found in user's PATH; not linting file.