[build] Allow other repos to control use_cxx23 with build_overrides

At least Angle will have to stay on C++20 after Chromium migrates to
C++23 [1], so give them an escape hatch. I'm hoping we can at least
get rid of use_cxx17 to avoid having 2 flags [2].

[1] https://crbug.com/330910097#comment15
[2] https://groups.google.com/a/google.com/g/angle-team/c/_SeqZIlTyAQ

Bug: 388070065
Change-Id: I5db734769953f71e51a1571964338a9d185b7a45
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7179482
Commit-Queue: Nico Weber <[email protected]>
Reviewed-by: Nico Weber <[email protected]>
Cr-Commit-Position: refs/heads/main@{#1548495}
NOKEYCHECK=True
GitOrigin-RevId: 79a63a62dad5cf0484983d94f83b0dcfc1f88283

Author: Victor Hugo Vianna Silva

config/compiler/compiler.gni

@@ -32,15 +32,20 @@ declare_args() {
   # The default linker everywhere else.
   use_lld = is_clang && current_os != "zos"
 
-  # If false, build using C++20; otherwise, build using C++23.
-  use_cxx23 = false
-
   # If true, optimize for size.
   # Default to favoring speed over size for platforms not listed below.
   optimize_for_size =
       !is_high_end_android && (is_android || is_castos || is_fuchsia)
 }
 
+# Allow other projects to keep use_cxx23 false by default if they aren't ready.
+if (!defined(use_cxx23)) {
+  declare_args() {
+    # If false, build using C++20; otherwise, build using C++23.
+    use_cxx23 = false
+  }
+}
+
 declare_args() {
   # Default to warnings as errors for default workflow, where we catch
   # warnings with known toolchains. Allow overriding this e.g. for Chromium