Fix metadirective and allocator parser regressions

[ouankou]

Summary

This promotes the parser fixes currently living on fix/ci-ctest-fragile-coverage
into ouankou/ompparser main so rex can pin the submodule to origin/main
instead of carrying a branch reference.

What changed

• fix structured BEGIN METADIRECTIVE / END METADIRECTIVE parsing so the end
  directive closes the block form rather than reusing the statement-associated
  metadirective production
• preserve mapper / allocator string payloads with owned std::string storage
  instead of transient raw pointers coming from parser actions
• reset clause-local string state explicitly before reuse so later clauses do not
  inherit stale parser state
• teach pragma regeneration to print the paired begin metadirective form

Why

These regressions showed up downstream in rex as parser and unparse failures in
fragile OpenMP coverage. Merging them into main makes the fix available through a
stable submodule target and removes the need to keep following a non-main branch.

Validation

• downstream rex builds against this branch
• downstream CI-focused ctest coverage includes the fragile OpenMP paths that
  originally exposed these regressions

[ouankou]

@codex review

[ouankou]

/gemini review

[gemini-code-assist]

Code Review

This pull request introduces support for the metadirective directive in the OpenMP parser and refactors the handling of string parameters by transitioning from const char * to std::string. The changes include updated grammar rules for metadirectives and improved safety through null checks and explicit string clearing. A critical feedback point was raised regarding the uses_allocators clause, where secondStringParameter was not being cleared for enum-based allocators, potentially leading to stale state from previous user-defined allocators.

[ouankou]

@codex review

[chatgpt-codex-connector]

Codex Review: Didn't find any major issues. Nice work!

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