Enable C++20 on Linux.

Doing this in advance of enabling for more platforms allows monitoring
compile-time effects with less chance of adversely affecting the CQ,
while still getting the benefit of breaking a bot if someone lands a
change that breaks the build only in C++20.

Bug: 1284275
Change-Id: Iff87b2114ad1be7c3f23dadc16f115f612d91d71
Cq-Include-Trybots: luci.chromium.try:linux-gcc-rel
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3932022
Reviewed-by: Andrew Grieve <agrieve@chromium.org>
Commit-Queue: Peter Kasting <pkasting@chromium.org>
Auto-Submit: Peter Kasting <pkasting@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1054983}

build/config/compiler/BUILD.gn

@@ -592,6 +592,15 @@ config("compiler") {
       if (is_clang) {
         cflags_cc += [ "-fno-trigraphs" ]
       }
+    } else if (is_linux) {
+      # TODO(crbug.com/1284275): Switch to C++20 on all platforms.
+      if (is_clang) {
+        cflags_cc += [ "-std=${standard_prefix}++20" ]
+      } else {
+        # The gcc bots are currently using GCC 9, which is not new enough to
+        # support "c++20"/"gnu++20".
+        cflags_cc += [ "-std=${standard_prefix}++2a" ]
+      }
     } else {
       cflags_cc += [ "-std=${standard_prefix}++17" ]
     }