Avoid MSVC warning D9025 when pdf_use_cxx20=false

When using MSVC and pdf_use_cxx20=false, we currently get lots of D9025
warnings because we pass both the /std:c++20 and /std:c++17 options. We
can avoid this warning by using the //build/config/compiler config's
"msvc_use_cxx17" variable when using MSVC (is_win && !is_clang), instead
of PDFium's //build_overrides/compiler:force_cxx17 config.

Bug: pdfium:1932
Change-Id: Ic42e76378acade2bf2baf472f076c7033fb1e50d
Reviewed-on: https://pdfium-review.googlesource.com/c/pdfium/+/102350
Commit-Queue: K. Moon <kmoon@chromium.org>
Reviewed-by: Lei Zhang <thestig@chromium.org>
Commit-Queue: Lei Zhang <thestig@chromium.org>
Auto-Submit: K. Moon <kmoon@chromium.org>

build_overrides/BUILDCONFIG.gn

diff --git a/build_overrides/BUILDCONFIG.gn b/build_overrides/BUILDCONFIG.gn
index c425b91..dce6fba 100644
--- a/build_overrides/BUILDCONFIG.gn
+++ b/build_overrides/BUILDCONFIG.gn
@@ -404,7 +404,11 @@
 # =============================================================================
 import("//pdfium.gni")
 if (!pdf_use_cxx20) {
-  default_compiler_configs += [ "//build_overrides/compiler:force_cxx17" ]
+  if (is_win && !is_clang) {
+    msvc_use_cxx17 = true
+  } else {
+    default_compiler_configs += [ "//build_overrides/compiler:force_cxx17" ]
+  }
 }
 
 # =============================================================================