Validate Enzyme pipeline versions

[krystophny]

Summary

• require matching LLVM major versions for Flang, opt, and llvm-link
• load-probe unversioned Enzyme plugins during configuration
• fail before compilation when the selected pipeline components are incompatible

Verification

Failing before on 189c6de:

$ cmake ... -DENZYME_LLVM_LINK_EXECUTABLE=<llvm-link-21>
-- Configuring done
-- Generating done

Passing after:

$ cmake ... -DENZYME_LLVM_LINK_EXECUTABLE=<llvm-link-21>
CMake Error at cmake/FindEnzyme.cmake:63 (MESSAGE):
  llvm-link LLVM 21 does not match opt LLVM 22

An invalid unversioned plugin likewise configured on the parent and is now rejected by the configure-time load probe. The matching LLVM 22 configuration built 149/149 targets and passed all four Enzyme tests.

$ fo
Static: OK (94 modules, 94 changed, 94 affected)
Build: OK
Tests: skipped, no affected tests
Lint: OK
All stages passed (.4s)