Define _LIBCPP_CONSTINIT when building libc++abi

The macro is defined in libc++, but at a more recent version than the one we
currently use. Define it ourselves for now.

Bug: 1298070
Change-Id: I84368fd309c916997637a7b49ded2144a264e162
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3468387
Reviewed-by: Nico Weber <thakis@chromium.org>
Commit-Queue: Nico Weber <thakis@chromium.org>
Cr-Commit-Position: refs/heads/main@{#972438}
NOKEYCHECK=True
GitOrigin-RevId: 3f516cfc18766fcb3d2308d2a043f6c40f5e0b4b
diff --git a/third_party/libc++abi/BUILD.gn b/third_party/libc++abi/BUILD.gn
index 1b0bea3..91839cc 100644
--- a/third_party/libc++abi/BUILD.gn
+++ b/third_party/libc++abi/BUILD.gn
@@ -60,6 +60,10 @@
   defines = [
     "LIBCXXABI_SILENT_TERMINATE",
     "_LIBCPP_ENABLE_CXX17_REMOVED_UNEXPECTED_FUNCTIONS",
+
+    # TODO(crbug.com/1298070) _LIBCPP_CONSTINIT is defined in a libc++ revision
+    # more recent than the one currently used in Chromium.
+    "_LIBCPP_CONSTINIT=constinit",
   ]
 
   configs -= [