[interop] Detect the resource dir of the required system LLVM

[aaronj0]

As discussed in the last meeting, this is the first step towards dropping LLVM as a dependency (required for the wheels and binary distributions) and ship the required clang headers. We need to be able to reliably detect the resource dir. Currently CreateInterpreter picks up any clang resource dir (naive clang probe) that will most likely not be the one that cppjit's libClangCppInterOp was built against

[github-actions]

Test Results

Configuration	Result
macos-26-intel-llvm21-py3.14-cxx20	= 437 passed, 44 skipped, 85 xfailed, 32 xpassed, 100 warnings in 90.18s (0:01:30) =
macos-26-llvm21-py3.14-cxx20	= 438 passed, 37 skipped, 93 xfailed, 30 xpassed, 100 warnings in 67.73s (0:01:07) =
ubuntu-24.04-llvm21-py3.14-cxx20-vg	====== 553 passed, 25 skipped, 18 xfailed, 2 xpassed in 82.69s (0:01:22) =======
ubuntu-24.04-llvm22-py3.14-cxx20	====== 553 passed, 25 skipped, 19 xfailed, 1 xpassed in 107.24s (0:01:47) ======
ubuntu-24.04-llvm22-py3.14-cxx20-cling	====== 543 passed, 25 skipped, 27 xfailed, 3 xpassed in 93.69s (0:01:33) =======
ubuntu-24.04-llvm22-py3.14-cxx23	====== 574 passed, 3 skipped, 20 xfailed, 1 xpassed in 107.22s (0:01:47) =======

[vgvassilev]

We need to develop some low cost "packaging" ci where we test for the common configurations; and also we make sure the tests run successfully on the package-installed system...

[aaronj0]

We need to develop some low cost "packaging" ci where we test for the common configurations; and also we make sure the tests run successfully on the package-installed system...

Something along those lines are done in #22, where a relocatable binary is shipped to arbitrary images with common configurations (manylinux, osx) and we reproduce whether installing the package and running tests pass. It's done in two steps, one host platform that builds the wheel and a target configuration that tests it.

I'll make sure to broaden the configs and keep the build cost low