Rollup merge of #127654 - nikic:llvm-ndebug-fix, r=cuviper

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`

compiler/rustc_codegen_llvm/src/llvm/ffi.rs

@@ -2057,7 +2057,7 @@ extern "C" {
         AddrOpsCount: c_uint,
         DL: &'a DILocation,
         InsertAtEnd: &'a BasicBlock,
-    ) -> &'a Value;
+    );
 
     pub fn LLVMRustDIBuilderCreateEnumerator<'a>(
         Builder: &DIBuilder<'a>,