Build icu with -Wno-unused-function.

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")
 
 # Meta target that includes both icuuc and icui18n. Most targets want both.
 # You can depend on the individually if you need to.
 group("icu") {
   deps = [
@@ skipping 49 matching lines @@
   } else if (is_linux) {
     cflags += [
       # Since ICU wants to internally use its own deprecated APIs, don't
       # complain about it.
       "-Wno-deprecated-declarations",
       "-Wno-unused-function",
     ]
   }
   if (is_clang) {
     cflags += [
+      # ICU uses its own deprecated functions.
       "-Wno-deprecated-declarations",
+      # ICU prefers `a && b || c` over `(a && b) || c`.
       "-Wno-logical-op-parentheses",
+      # ICU has some `unsigned < 0` checks.
       "-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.
+      "-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://bugs.icu-project.org/trac/ticket/11757.
         "-Wno-reorder",
       ]
     }
@@ skipping 480 matching lines @@
         sources = [
           "android/icudtl_dat.S",
         ]
       } else {
         assert(false, "No icu data for this platform")
       }
       defines = [ "U_HIDE_DATA_SYMBOL" ]
     }
   }
 }