[heap] Change MarkingTreatmentMode to an enum class

This fixes a -Wshadow warning with an inline enum in regexp-compiler.cc.

Bug: v8:12244,v8:12245
Change-Id: I8b53a94a1945addb958b230abe01b10d4533edae
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3285732Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Zhi An Ng <zhin@chromium.org>
Cr-Commit-Position: refs/heads/main@{#77935}

src/heap/mark-compact.h

@@ -186,7 +186,7 @@ class LiveObjectVisitor : AllStatic {
 
 enum class AlwaysPromoteYoung { kYes, kNo };
 enum PageEvacuationMode { NEW_TO_NEW, NEW_TO_OLD };
-enum MarkingTreatmentMode { KEEP, CLEAR };
+enum class MarkingTreatmentMode { KEEP, CLEAR };
 enum class RememberedSetUpdatingMode { ALL, OLD_TO_NEW_ONLY };
 
 // Base class for minor and full MC collectors.