Verify llvm-needs-components are not empty and match the --target value
I recently discovered a test with an empty `llvm-needs-components` entry (fixed in rust-lang/rust#143979) which meant that it didn't work correctly when building Rust with a limited set of LLVM targets.
This change makes a pair of improvements to prevent this issue from creeping in again:
* When parsing directives with values, `compiletest` will now raise an error if there is an empty value.
* Improved the `target_specific_tests` tidy checker to map targets to LLVM components, to verify that any existing `llvm-needs-components` contains the target being used.
I also fixed all the issues flagged by the improved tidy checker.
b6ac83d372