[ci] Consolidate the OOP-JIT and in-process LLVM caches.

[vgvassilev]

Two OOP-JIT cells (ubu24-x86-gcc12-llvm22-oop-vg, osx26-arm-clang-llvm22-oop) used a dedicated -oop-suffixed cache slot. Build_LLVM baked llvm-jitlink-executor and orc_rt-<arch> into that slot under a matrix.oop-jit == 'On' gate, and the matrix rows additionally enabled
llvm_enable_projects: clang;compiler-rt so the orc_rt target existed in the LLVM build. The whole arrangement existed because OOP-JIT used to require clang20-1-out-of-process.patch (a 966-line backport of clang::Interpreter::JITConfig), which genuinely changed the cached binary.

That patch retired in fb595ac (#924) when OOP-JIT moved to upstream release/22.x's clang::IncrementalExecutorBuilder. The OOP and in-process LLVM builds are now identical modulo the two runtime-only artifacts (llvm-jitlink-executor and orc_rt). Drop the dedicated cache slot and source the runtime parts from packages instead, so OOP rows free-ride the same cache as their in-process counterparts.

Cache-side:

• main.yml: drop the ${{ matrix.oop-jit == 'On' && '-oop' || '' }} segment from the cache key. Every non-OOP key is bit-identical to before, so existing cache entries continue to hit -- no invalidation. The previously -oop-suffixed entries become orphaned (still in storage until eviction, just unreferenced).
• main.yml matrix: drop llvm_enable_projects: clang;compiler-rt from both OOP rows so they share build config, and therefore cache content, with their non-OOP twins on the same axes.
• Build_LLVM/action.yml: drop the if oop-jit == On: ninja llvm-jitlink-executor orc_rt-<suffix> block. The cache now contains a vanilla LLVM build regardless of matrix.oop-jit.

Runtime-side, OOP coverage preserved:

• main.yml adds an Install OOP-JIT runtime step gated on matrix.oop-jit == 'On'. It installs the runtime parts via apt.llvm.org (llvm-${runtime} for the executor binary, libclang-rt-${runtime}-dev for the orc_rt static archive) on Linux, or brew install llvm@${runtime} on macOS. The install step then symlinks both into the test binary's expected resource-dir layout (<test-bin-dir>/lib/clang/<major>/{bin,lib/<triple>}/). That path matters: at test time clang::IncrementalExecutor Builder derives the resource directory from getMainExecutable() (the test binary), and the upstream safety check refuses to use an orc_rt whose path isn't under that directory. Symlinking before Build_and_Test_ CppInterOp runs is fine -- CppInterOp's build doesn't overwrite those paths.
• Build_and_Test_CppInterOp/action.yml keeps both the -DLLVM_BUILT_WITH_OOP_JIT=${{ matrix.oop-jit }} cmake flag and the OOP-specific Valgrind suppression branch. The OOP test fixtures from PR Out-Of-Process Interpreter for CppInterOp #717 still parameterize over both in-process and OOP modes on those rows.

The cached LLVM stays LLVM_ENABLE_ASSERTIONS=ON, so the JIT compiler that runs in-process during CppInterOp's tests still fires its internal assertions -- which is the assertion signal worth keeping. orc_rt and llvm-jitlink-executor are runtime support code, not assertion-rich, so sourcing them from packages doesn't lose coverage.

Two follow-ups out of scope for this commit:

• Retire the LLVM_BUILT_WITH_OOP_JIT macro from CppInterOp source. The compile-time gate becomes an LLVM-version / platform check, and a runtime check decides whether to actually use OOP. After that, Build_and_Test_CppInterOp no longer needs the cmake flag and the matrix's oop-jit field becomes purely a Valgrind-mode hint.
• Bundle orc_rt (and ship llvm-jitlink-executor) inside libclangCppInterOp.so's distribution so the library is self-contained. Removes the install step entirely.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 80.87%. Comparing base (fb595ac) to head (22aed11).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #939   +/-   ##
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  Coverage   80.87%   80.87%           
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  Files          15       15           
  Lines        4707     4707           
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  Hits         3807     3807           
  Misses        900      900           

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