clc: Declare LLVMContexts on the stack

clc: Declare LLVMContexts on the stack

This prevents more use-after-free errors.  Passing them around using
std::unique_ptr ensures that the LLVMContext gets destroyed but doesn't
ensure destruction order.  Declaring it on the stack ensures that the
context doesn't get destroyed until right before the the function
returns which is after any other LLVM stuff is destroyed.

Reviewed-by: Jesse Natalie <jenatali@microsoft.com>
Reviewed-by: Icecream95 <ixn@disroot.org>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/15937>