Build icu with -Wno-unused-function.

BUILD.gn

Index: BUILD.gn
diff --git a/BUILD.gn b/BUILD.gn
index e56e07b75452b1b5e10711f4a24c222442051f86..0378eb6677d41c1a8a1db121ff3ae59bb0b4312b 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -67,16 +67,29 @@ config("icu_code") {
   }
   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 += [