[gcc r15-8007] libstdc++: Use new <bits/monostate.h> header in <format>

[Jonathan Wakely]

https://gcc.gnu.org/g:d8a3944b440777d2d6fb4a9f456189a0dc439ec8

commit r15-8007-gd8a3944b440777d2d6fb4a9f456189a0dc439ec8
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Wed Mar 12 17:43:08 2025 +0000

    libstdc++: Use new <bits/monostate.h> header in <format>
    
    In r15-5832-g91f4550e1700b7 std::monostate was moved to its own header,
    which means that <format> no longer needs to include all of <variant>.
    
    libstdc++-v3/ChangeLog:
    
            * include/std/format: Include <bits/monostate.h> instead of
            <variant>.

Diff:
---
 libstdc++-v3/include/std/format | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/libstdc++-v3/include/std/format b/libstdc++-v3/include/std/format
index bc26599b9f97..f52645a3ac75 100644
--- a/libstdc++-v3/include/std/format
+++ b/libstdc++-v3/include/std/format
@@ -51,7 +51,7 @@
 #include <span>
 #include <string_view>
 #include <string>
-#include <variant>	       // monostate (TODO: move to bits/utility.h?)
+#include <bits/monostate.h>
 #include <bits/ranges_base.h>  // input_range, range_reference_t
 #include <bits/ranges_util.h>  // subrange
 #include <bits/ranges_algobase.h> // ranges::copy