[interop] Resolve libclangCppInterOp relative to libcppjit at runtime

[aaronj0]

The backend used to locate libclangCppInterOp through absolute build-time paths, so a wheel built on one machine could not load on another.

This PR implements the following:

• Library and headers are resolved relative to libcppjit's own load location, with the build-time install prefix as the fallback for build trees. This is necessary for the wheels building infrastructure (Support building wheels for manylinux and OSX #22).

Validation:

• full test suite passes locally across different cxx standard builds
• wheel relocatability: a wheel built in an isolated environment installs and runs in a second fresh venv after the build environment is deleted
• the install-prefix fallback verified against a bare build-directory libcppjit.so
• Ran clang-format-18

[github-actions]

Test Results

Configuration	Result
macos-26-intel-llvm21-py3.14-cxx20	= 437 passed, 44 skipped, 85 xfailed, 32 xpassed, 100 warnings in 111.23s (0:01:51) =
macos-26-llvm21-py3.14-cxx20	= 438 passed, 37 skipped, 93 xfailed, 30 xpassed, 100 warnings in 62.02s (0:01:02) =
ubuntu-24.04-llvm21-py3.14-cxx20-vg	=========== 553 passed, 25 skipped, 18 xfailed, 2 xpassed in 59.35s ============
ubuntu-24.04-llvm22-py3.14-cxx20	====== 553 passed, 25 skipped, 19 xfailed, 1 xpassed in 108.50s (0:01:48) ======
ubuntu-24.04-llvm22-py3.14-cxx20-cling	====== 543 passed, 25 skipped, 27 xfailed, 3 xpassed in 103.52s (0:01:43) ======
ubuntu-24.04-llvm22-py3.14-cxx23	====== 574 passed, 3 skipped, 20 xfailed, 1 xpassed in 117.26s (0:01:57) =======

[Vipul-Cariappa]

LGTM! Modulo Vassil's comment.