Partially revert a535042e80
Even with non-LLVM codegen backends, we want to allow for annotations
that express dependencies to LLVM-specific parts of the test suite.
This includes `//@ needs-llvm-components`, which just allows checking
that LLVM is built with relevant target support before the test is run.
It does not assert the test cannot work with another codegen backend.
99550fbc3e