Better diagnostics for dlltool errors.

When dlltool fails, show the full command that was executed. In
particular, llvm-dlltool is not very helpful, printing a generic usage
message rather than what actually went wrong, so stdout and stderr
aren't of much use when troubleshooting.

compiler/rustc_codegen_llvm/src/errors.rs

@@ -81,6 +81,8 @@ pub(crate) struct ErrorCallingDllTool<'a> {
 #[derive(Diagnostic)]
 #[diag(codegen_llvm_dlltool_fail_import_library)]
 pub(crate) struct DlltoolFailImportLibrary<'a> {
+    pub dlltool_path: Cow<'a, str>,
+    pub dlltool_args: String,
     pub stdout: Cow<'a, str>,
     pub stderr: Cow<'a, str>,
 }