[build] Extract C++-only warning flags in GN compiler configs

Several C++-only warning suppressions were incorrectly placed in the
shared 'default_warning_flags' list instead of
'default_warning_flags_cc'. This caused these flags (such as
-Wno-microsoft-unqualified-friend, -Wno-microsoft-cast)
to be applied to pure C targets (like zlib and BoringSSL assembly).

This CL moves these C++-only warnings to 'default_warning_flags_cc',
ensuring they are only applied to C++ compilation units. This improves
compile database accuracy, prevents IDE/clangd from surfacing phantom
diagnostics on C files, and makes the build configuration explicit and
robust.

Change-Id: I117de03496164d7e2e39b5deab7754fd85452923
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/510181
Commit-Queue: Kevin Moore <kevmoo@google.com>
Reviewed-by: Ryan Macnak <rmacnak@google.com>
Auto-Submit: Kevin Moore <kevmoo@google.com>

build/config/compiler/BUILD.gn

@@ -639,11 +639,13 @@ if (is_win) {
       "-Wno-ignored-pragma-optimize",  # icu, double-conversion
       "-Wno-macro-redefined",
       "-Wno-microsoft-cast",
-      "-Wno-microsoft-unqualified-friend",
       "-Wno-unknown-argument",  # icu
       "-Wno-unused-value",  # crashpad
       "-Wno-deprecated-non-prototype",  # zlib
     ]
+    default_warning_flags_cc += [
+      "-Wno-microsoft-unqualified-friend",
+    ]
   } else {
     default_warning_flags += [
       # Permanent.