Add support for MC/DC coverage (#363)

Co-authored-by: Taiki Endo <[email protected]>

Author: aytey

.github/.cspell/project-dictionary.txt

@@ -10,6 +10,7 @@ fprofile
 instrprof
 libclang
 libhello
+mcdc
 microkernel
 MSYSTEM
 nextest

src/main.rs

@@ -1012,6 +1012,16 @@ impl Format {
                     "-show-line-counts-or-regions",
                     "-show-expansions",
                     "-show-branches=count",
+                ]);
+                if cmd!(&cx.llvm_cov, "show", "--help")
+                    .read()
+                    .unwrap_or_default()
+                    .contains("-show-mcdc")
+                {
+                    // -show-mcdc requires LLVM 18+
+                    cmd.arg("-show-mcdc");
+                }
+                cmd.args([
                     &format!("-Xdemangler={}", cx.current_exe.display()),
                     "-Xdemangler=llvm-cov",
                     "-Xdemangler=demangle",