contrib: Use -Xdemangler=llvm-cxxfilt in deterministic-*-coverage

This makes the result more readable.

contrib/devtools/deterministic-unittest-coverage/src/main.rs

@@ -104,6 +104,8 @@ fn deterministic_coverage(build_dir: &Path, test_exe: &Path, filter: &str) -> Ap
                 "--show-line-counts-or-regions",
                 "--show-branches=count",
                 "--show-expansions",
+                "--show-instantiation-summary",
+                "-Xdemangler=llvm-cxxfilt",
                 &format!("--instr-profile={}", profdata_file.display()),
             ])
             .arg(test_exe)