Refactor OpenACC clause parsing/unparsing

[ouankou]

Summary

• Refactor clause parsing/IR to eliminate raw-string payloads: var-lists, async/default_async, device/device_num/num_gangs/num_workers/collapse/tile, gang/wait/bind/routine, data-like clauses, reductions, self, cache, etc. now carry structured fields/enums in OpenACCKinds.h and AST rather than opaque strings; clause merging for self disabled.
• Clean up unparsing: generatePragmaString joins clauses without trim hacks; expression/var-list formatting avoids trailing comma trims; reduction emits reduction(op : vars); routine/gang/wait/bind/copy* formatting normalized; removed after-the-fact spacing/comma hacks.
• Testing: test_single_pragma.sh enforces roundtrip comparisons; preprocess keeps inputs intact; lexer now accepts comma-separated clauses via CLAUSE_COMMA so users need no preprocessing; canonical output remains space-separated. All ctests (including openacc_vv roundtrip) passing.

Testing

• cmake --build build
• ctest --output-on-failure