api: Fix constexpr construction of compaction space index

Bug: chromium:1056170
Change-Id: If639b12e1cceec2d27355bb4cbf8c1fefa8b5038
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2642462
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Omer Katz <omerkatz@chromium.org>
Auto-Submit: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Omer Katz <omerkatz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#72283}

include/cppgc/custom-space.h

@@ -9,8 +9,11 @@
 
 namespace cppgc {
 
+/**
+ * Index identifying a custom space.
+ */
 struct CustomSpaceIndex {
-  CustomSpaceIndex(size_t value) : value(value) {}  // NOLINT
+  constexpr CustomSpaceIndex(size_t value) : value(value) {}  // NOLINT
   size_t value;
 };