Make is_win_fastlink imply -fstandalone-debug for clang

build/config/compiler/BUILD.gn

Index: build/config/compiler/BUILD.gn
diff --git a/build/config/compiler/BUILD.gn b/build/config/compiler/BUILD.gn
index f3a69627b3b04726b55af755978481a6a7cd2ff7..b5231b1537587a51bdcf520f1adb0afc7eb91cda 100644
--- a/build/config/compiler/BUILD.gn
+++ b/build/config/compiler/BUILD.gn
@@ -1560,6 +1560,16 @@ config("symbols") {
     } else {
       ldflags = [ "/DEBUG" ]
     }
+
+    if (is_clang) {
+      # /DEBUG:FASTLINK requires every object file to have standalone debug
+      # information.
+      if (is_win_fastlink) {
+        cflags += [ "-fstandalone-debug" ]
+      } else {
+        cflags += [ "-fno-standalone-debug" ]
+      }
+    }
   } else {
     if (is_mac || is_ios) {
       cflags = [ "-gdwarf-2" ]