[gn] add a bunch of missing libc++ headers

Ports:
* fcaccf8 (mdspan)
* 73d94b1 (source_location)
* b77e50e (stop_token)
* 7cc72a0 (syncstream)

Author: nico

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

@@ -1021,6 +1021,7 @@ if (current_toolchain == default_toolchain) {
       "locale.h",
       "map",
       "math.h",
+      "mdspan",
       "memory",
       "memory_resource",
       "mutex",
@@ -1039,6 +1040,7 @@ if (current_toolchain == default_toolchain) {
       "semaphore",
       "set",
       "shared_mutex",
+      "source_location",
       "span",
       "sstream",
       "stack",
@@ -1049,11 +1051,13 @@ if (current_toolchain == default_toolchain) {
       "stdint.h",
       "stdio.h",
       "stdlib.h",
+      "stop_token",
       "streambuf",
       "string",
       "string.h",
       "string_view",
       "strstream",
+      "syncstream",
       "system_error",
       "tgmath.h",
       "thread",