Backport another libc++ patch for std::filesystem (and locale) usage

This fixes linking against a DLL of libc++ while using the common headers
(generated from the static version of the library, but which work for
both shared and static versions) when calling e.g. std::filesystem::u8path.

patches/llvm-project/0013-libcxx-Implement-is_absolute-properly-for-windows.patch

@@ -1,7 +1,7 @@
 From bd50990c64deae4ce6027767659c75a22822e2f5 Mon Sep 17 00:00:00 2001
 From: =?UTF-8?q?Martin=20Storsj=C3=B6?= <martin@martin.st>
 Date: Sun, 1 Nov 2020 23:39:03 +0200
-Subject: [PATCH 13/24] [libcxx] Implement is_absolute properly for windows
+Subject: [PATCH 13/25] [libcxx] Implement is_absolute properly for windows
 
 Differential Revision: https://reviews.llvm.org/D91177