fix: revert cxx_scan restat (P2) — broke self-host smoke test

The $out.tmp + cmp -s approach for content-stable .ddi writes caused
linker failures in the CI self-host smoke step. GCC's -fdeps-file=
interacts with ninja's depfile tracking in ways that make the temp-file
approach fragile. Revert to direct -fdeps-file=$out for now.

P1 (fingerprint coherence) and P1.5 (diagnostic warning) are retained.
P2 needs a different approach — possibly ninja's built-in restat alone
without temp-file wrappers.

Author: Sunrisepeak

src/build/ninja_backend.cppm

@@ -243,36 +243,21 @@ std::string emit_ninja_string(const BuildPlan& plan) {
 
     if (dyndep) {
         // Scan rule: produce P1689 .ddi for one TU.
-        // P2: write to $out.tmp first, then compare with existing $out.
-        // If content is identical, keep old $out (preserving mtime) so
-        // downstream dyndep/compile edges are not marked dirty.
-        // restat = 1 tells Ninja to check if $out actually changed.
         // GCC: built-in -fdeps-format=p1689r5 flags during preprocessing.
         // Clang: external clang-scan-deps tool with -format=p1689.
         append("rule cxx_scan\n");
         if (plan.scanDepsPath.empty()) {
             // GCC path: compiler-integrated P1689 scanning.
             append("  command = $toolenv $cxx $cxxflags -fmodules "
                    "-fdeps-format=p1689r5 "
-                   "-fdeps-file=$out.tmp -fdeps-target=$compile_target "
-                   "-M -MM -MF $out.dep -E $in -o $compile_target && "
-                   "if [ -f \"$out\" ] && cmp -s \"$out.tmp\" \"$out\"; then "
-                     "rm -f \"$out.tmp\"; "
-                   "else "
-                     "mv -f \"$out.tmp\" \"$out\"; "
-                   "fi\n");
+                   "-fdeps-file=$out -fdeps-target=$compile_target "
+                   "-M -MM -MF $out.dep -E $in -o $compile_target\n");
         } else {
             // Clang path: clang-scan-deps produces P1689 JSON to stdout.
             append("  command = $toolenv $scan_deps -format=p1689 -- "
-                   "$cxx $cxxflags -c $in -o $compile_target > $out.tmp && "
-                   "if [ -f \"$out\" ] && cmp -s \"$out.tmp\" \"$out\"; then "
-                     "rm -f \"$out.tmp\"; "
-                   "else "
-                     "mv -f \"$out.tmp\" \"$out\"; "
-                   "fi\n");
+                   "$cxx $cxxflags -c $in -o $compile_target > $out\n");
         }
-        append("  description = SCAN $out\n");
-        append("  restat = 1\n\n");
+        append("  description = SCAN $out\n\n");
 
         // Aggregate .ddi files into a Ninja dyndep file.
         append(std::format(