[CMake] `find_package` call destroys `LLVM_DIR` variable

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Describe the bug

Hi,

I've set the LLVM_DIR variable to specify a (custom/local) LLVM build and I noticed two issues:

• The call find_package(LLVM ${CUDAQ_LLVM_VERSION} CONFIG QUIET) destroys the variable and shows as not found in later stages.
• The QUIET in the previous call actually suppressed any underlying cause of failure and should probably be more user friendly. In fact, if the initial attempt at LLVM resolution fails, the build needs to put a warning so that the user is made aware.

Best,

Steps to reproduce the bug

• Simply surround the call find_package() with status messages:

message(STATUS "LLVM_DIR Before the call: ${LLVM_DIR}")
find_package(LLVM ${CUDAQ_LLVM_VERSION} CONFIG QUIET)
message(STATUS "LLVM_DIR After the call: ${LLVM_DIR}")

• Run CMake config with -DLLVM_DIR="/some/path" or via a preset file (I use the latter)

Expected behavior

I expect the LLVM_DIR variable to remain intact after the call find_project() as it's gonna be needed for later stages

Is this a regression? If it is, put the last known working version (or commit) here.

Not a regression

Environment

• CUDA-Q version: main branch
• Python version: 3.11
• C++ compiler: Clang
• Operating system: Arch Linux

Suggestions

Maybe reorder LLVM resolution steps. Definitely revisit the call find_package(). I don't think it should be QUIET. LLVM resolution is really painful, I have a feeling it's too overfit. I would make it simple: be explicit about expected versions and support custom builds (of those expected versions)

Start at the CMake entry point containing find_package(LLVM ${CUDAQ_LLVM_VERSION} CONFIG QUIET) and inspect the surrounding LLVM resolution steps. Reproduce with CMake configured using -DLLVM_DIR="/some/path" or a preset, then verify that LLVM_DIR remains available afterward and that an unsuccessful initial resolution gives a visible warning.