[gn build] Port 0876668

Author: llvmgnsyncbot

llvm/utils/gn/secondary/libcxx/include/BUILD.gn

@@ -450,7 +450,6 @@ if (current_toolchain == default_toolchain) {
       "__format/format_context.h",
       "__format/format_error.h",
       "__format/format_functions.h",
-      "__format/format_fwd.h",
       "__format/format_parse_context.h",
       "__format/format_string.h",
       "__format/format_to_n_result.h",
@@ -500,6 +499,7 @@ if (current_toolchain == default_toolchain) {
       "__fwd/array.h",
       "__fwd/bit_reference.h",
       "__fwd/complex.h",
+      "__fwd/format.h",
       "__fwd/fstream.h",
       "__fwd/functional.h",
       "__fwd/ios.h",