Return to suppressing deprecation warnings

BUILD.gn

Index: BUILD.gn
diff --git a/BUILD.gn b/BUILD.gn
index 79a31f045bde3f309ecce597d0e967b72a9b0c8e..046ec21387e62a04797ae4c1846bc9face741444 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -82,6 +82,8 @@ config("icu_code") {
   }
   if (is_clang) {
     cflags += [
+      # ICU uses its own deprecated functions.
+      "-Wno-deprecated-declarations",
       # ICU has some code with the pattern:
       #   if (found = uprv_getWindowsTimeZoneInfo(...))
       "-Wno-parentheses",