Fix incorrect NDEBUG handling in LLVM bindings
We currently compile our LLVM bindings using `-DNDEBUG` if debuginfo for LLVM is disabled. However, `NDEBUG` doesn't have any relation to debuginfo, it controls whether assertions are enabled.
Split the LLVM_NDEBUG environment variable into two, so that assertions and debuginfo are controlled independently.
After this change, `LLVMRustDIBuilderInsertDeclareAtEnd` triggers an assertion failure on LLVM 19 due to an incorrect cast. Fix it by removing the unused return value entirely.
r? `@cuviper`
cb1ccc1842