fix(deps): only require LLVM when building the LLVM pass

[w00tzenheimer]

Problem

dependencies/CMakeLists.txt includes llvm.cmake unconditionally. With the
default USE_EXTERNAL_LLVM=ON that reaches
find_package(LLVM REQUIRED CONFIG) at dependencies/llvm.cmake:8, so every
dependency build fails unless a system LLVM is installed. Setting
USE_EXTERNAL_LLVM=OFF avoids that only by building LLVM from source.

Neither is needed for cobra-core, which references zero LLVM symbols:

$ nm -u build/lib/core/libcobra-core.a | grep -c -i llvm
0

LLVM is used only by lib/llvm, which the main project already gates on
COBRA_BUILD_LLVM_PASS (CMakeLists.txt:64). The superbuild was missing the
same gate.

Fix

Gate include(llvm.cmake) on COBRA_BUILD_LLVM_PASS, matching the main
project. Default is unchanged (OFF), so builds that do want the pass plugin
behave exactly as before by passing -DCOBRA_BUILD_LLVM_PASS=ON.

Verification

Counterfactual, with LLVM hidden from CMake. Before the change:

CMake Error at llvm.cmake:8 (find_package):
  Could NOT find LLVM (missing: LLVM_DIR)
Call Stack (most recent call first):
  CMakeLists.txt:31 (include)

After the change the same command configures and builds cleanly:

cmake -S dependencies -B build-deps -DCMAKE_BUILD_TYPE=Release
cmake --build build-deps
cmake -S . -B build -DCMAKE_BUILD_TYPE=Release \
  -DCMAKE_PREFIX_PATH=$(pwd)/build-deps/install
cmake --build build
./build/tools/cobra-cli/cobra-cli --mba "(x|y)-(x&y)" --bitwidth 32
x ^ y

The dependency prefix contains no LLVM afterwards, and cobra-cli links only
CoreFoundation, libc++ and libSystem.

Verified on macOS arm64, macOS x86_64 and Linux (manylinux_2_28). Windows
additionally needs the companion MSVC fix, since cobra-core does not compile
under MSVC today.

Motivation

This makes it possible to build cobra-core inside cibuildwheel images for
Python bindings, where installing a full LLVM is not practical.

[CLAassistant]

All committers have signed the CLA.

[kyle-elliott-tob]

Will merge after the CLA has been signed, thanks :)

[w00tzenheimer]

Done!