Hoist Python compiler discovery and probe compile helpers

[doublemover]

Parent inventory: #8108
Blocked by: #8119, #8120, #8113

Objective

Centralize compiler discovery and probe compile/run command construction for runtime acceptance and runnable e2e scripts.

Shared Module Specification

• Add scripts/objc3c_tooling/probe_compile.py.
• Expose Windows-aware find_clangxx behavior with LLVM_ROOT/PATH discovery where currently supported.
• Expose compile command construction with production include root and explicit -I tests/tooling/runtime support include root.
• Support extra object inputs, runtime shims, output executable path, and bounded diagnostics.
• Reuse shared subprocess helpers for duration and failure reporting.

Migration Checklist

• Migrate runnable find_clangxx copies.
• Migrate scripts/check_objc3c_runtime_acceptance.py:1334 compiler discovery if dependency direction stays clean.
• Migrate runnable compile_probe copies.
• Migrate scripts/check_objc3c_runtime_acceptance.py:5173 compile helper if all special cases are preserved.
• Keep compile behavior identical except for explicit support include root.

Complete Replacement Inventory

• find_clangxx - 11 definition(s)/local occurrence(s) to replace or explicitly document as intentionally local if semantics differ.
  • scripts/check_objc3c_runnable_block_arc_end_to_end.py:97
  • scripts/check_objc3c_runnable_bonus_experience_end_to_end.py:80
  • scripts/check_objc3c_runnable_concurrency_end_to_end.py:108
  • scripts/check_objc3c_runnable_error_end_to_end.py:97
  • scripts/check_objc3c_runnable_interop_end_to_end.py:112
  • scripts/check_objc3c_runnable_metaprogramming_end_to_end.py:112
  • scripts/check_objc3c_runnable_object_model_end_to_end.py:97
  • scripts/check_objc3c_runnable_release_candidate_end_to_end.py:110
  • scripts/check_objc3c_runnable_showcase_end_to_end.py:80
  • scripts/check_objc3c_runnable_storage_reflection_end_to_end.py:97
  • scripts/check_objc3c_runtime_acceptance.py:1334
• compile_probe - 5 definition(s)/local occurrence(s) to replace or explicitly document as intentionally local if semantics differ.
  • scripts/check_objc3c_runnable_concurrency_end_to_end.py:157
  • scripts/check_objc3c_runnable_interop_end_to_end.py:166
  • scripts/check_objc3c_runnable_metaprogramming_end_to_end.py:163
  • scripts/check_objc3c_runnable_release_candidate_end_to_end.py:122
  • scripts/check_objc3c_runtime_acceptance.py:5173

Performance And Reliability Requirements

• Keep helper code small, dependency-light, and fast enough for normal development feedback.
• Add focused tests for edge cases before broad migration.
• Preserve existing output schemas and public behavior unless an intentional bug fix is validated.
• Keep production helpers separated from test-only helpers.
• Do not use tmp/ as durable source-of-truth; generated temporary files are allowed only as transient command inputs.

Fast Tests

• Add tests for compiler discovery priority and missing compiler diagnostics.
• Add tests for compile command construction including support include root and extra objects.
• Run focused runnable e2e probes for concurrency, interop, metaprogramming, release candidate, and runtime acceptance probe cases.
• Run python -m py_compile for changed modules/scripts.

End-To-End Completion Criteria

• All semantically equivalent compiler discovery and probe compile helpers are shared.
• Future include paths/warning flags/runtime object inputs can be adjusted in one place.
• Probe compiles remain deterministic and fast to diagnose.