Use -Wno-unused-const-variable only on Windows + clang

BUILD.gn

 # Copyright 2014 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 import("//third_party/icu/config.gni")
 
 if (is_android) {
   import("//build/config/android/rules.gni")
 }
 
@@ skipping 83 matching lines @@
       "-Wno-tautological-compare",
       # uresdata.c has switch(RES_GET_TYPE(x)) code. The
       # RES_GET_TYPE macro returns an UResType enum, but some switch
       # statement contains case values that aren't part of that
       # enum (e.g. URES_TABLE32 which is in UResInternalType). This
       # is on purpose.
       "-Wno-switch",
       # ICU has some code with the pattern:
       #   if (found = uprv_getWindowsTimeZoneInfo(...))
       "-Wno-parentheses",
-      # ICU generally has no unused variables, but there are a few places where
-      # this warning triggers.
-      # See https://codereview.chromium.org/1222643002/ and
-      # http://www.icu-project.org/trac/ticket/11759.
-      "-Wno-unused-const-variable",
       # ucnv2022.cpp contains three functions that are only used when
       # certain preprocessor defines are set.
+      # unistr.cpp also has an unused function for non-component builds.
       "-Wno-unused-function",
     ]
     if (is_win) {
       cflags += [
         # See http://bugs.icu-project.org/trac/ticket/11122
         "-Wno-inline-new-delete",
         "-Wno-implicit-exception-spec-mismatch",
+        # See http://www.icu-project.org/trac/ticket/11759.
+        "-Wno-unused-const-variable",
       ]
     }
   }
 }
 
 component("icui18n") {
   # find  source/i18n -maxdepth 1  ! -type d  | egrep  '\.(c|cpp|h)$' |\
   # sort | sed 's/^\(.*\)$/    "\1",/'
   sources = [
     "source/i18n/affixpatternparser.cpp",
@@ skipping 913 matching lines @@
       }
     }
 
     source_set("icudata") {
       sources = [ "$data_assembly" ]
       defines = [ "U_HIDE_DATA_SYMBOL" ]
       deps = [ ":make_data_assembly", ]
     }
   }
 }