Update WSM format to be OS-agnostic

The warning suppression mapping is apparently sensitive to which path
separator is used, and it isn't always consistent. To make sure we
properly suppress things on windows, update the entries to match and
combination of the separators.

This CL also re-enables the warning on windows, since we're now suppressing it properly via the WSM.

Bug: 425364464
Change-Id: Ie7804e3b02bdc144987492634250018629bf77e7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6653054
Reviewed-by: Arthur Eubanks <aeubanks@google.com>
Commit-Queue: Devon Loehr <dloehr@google.com>
Cr-Commit-Position: refs/heads/main@{#1475806}
NOKEYCHECK=True
GitOrigin-RevId: 26d51346374a0d16b0ba2243ef83c015a944d975

Author: DKLoehr

config/compiler/BUILD.gn

@@ -2161,7 +2161,7 @@ config("default_warnings") {
       ]
     }
 
-    if (is_component_build && use_clang_warning_suppression_file && !is_win) {
+    if (is_component_build && use_clang_warning_suppression_file) {
       # Warn for globally unique objects which might get duplicated if built
       # into different components, due to hidden visibility. Only applicable
       # to component builds.

config/warning_suppression.txt

@@ -29,16 +29,19 @@
 # We may make an exception to the policy for extremely high-value warnings that backslide a lot
 # (such as unsafe buffers), but this is expected to be rare.
 
-# Formatting note: Don't put comments on the same line as a glob pattern! Clang
-# will get confused and the warning won't be suppressed.
+# Formatting Notes:
+# 1. Don't put comments on the same line as a glob pattern! Clang will get confused,
+#    and the warning won't be suppressed.
+# 2. The syntax {/,\\} matches either a backslash or forward slash. They can sometimes
+#    be intermixed in a single path when compiling on Windows.
 
 [unnecessary-virtual-specifier]
 # Can be removed when https://github.com/google/nearby/pull/3392 is merged and rolled
-src:*/third_party/nearby/*
+src:*{/,\\}third_party{/,\\}nearby{/,\\}*
 
 [unique-object-duplication]
 # This warning is extremely difficult to fix in code we don't directly control.
 # As an experiment, we're breaking the policy and suppressing it permanently.
-src:*/third_party/*
+src:*{/,\\}third_party{/,\\}*
 # v8's build system isn't amenable to fixes: https://crbug.com/408259026
-src:*/v8/*
+src:*{/,\\}v8{/,\\}*