Enable bugprone-forward-declaration-namespace clang-tidy check

Following this discussion on cxx[1], this CL enables
bugprone-forward-declaration-namespace clang-tidy check in Tricium.

[1] https://groups.google.com/a/chromium.org/g/cxx/c/guBh6Nlv2Nk

Bug: 1519029
Change-Id: I25ee6147d47951556ab641b7ca5157e4f7bae75f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5203385
Reviewed-by: danakj <danakj@chromium.org>
Reviewed-by: Peter Kasting <pkasting@chromium.org>
Commit-Queue: Miyoung Shin <myid.shin@igalia.com>
Cr-Commit-Position: refs/heads/main@{#1267514}

.clang-tidy

@@ -6,6 +6,7 @@
                     bugprone-assert-side-effect,
                     bugprone-bool-pointer-implicit-conversion,
                     bugprone-dangling-handle,
+                    bugprone-forward-declaration-namespace,
                     bugprone-inaccurate-erase,
                     bugprone-string-constructor,
                     bugprone-string-integer-assignment,

base/win/.clang-tidy

@@ -2,6 +2,7 @@
 Checks: '-*,
          bugprone-argument-comment,
          bugprone-bool-pointer-implicit-conversion,
+         bugprone-forward-declaration-namespace,
          bugprone-unique-ptr-array-mismatch,
          bugprone-virtual-near-miss,
          google-build-namespaces,