Enable -Wcovered-switch-default as it matches the switch style used in llvm.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@151609 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/autoconf/configure.ac b/autoconf/configure.ac
index 3a86531..b28508e 100644
--- a/autoconf/configure.ac
+++ b/autoconf/configure.ac
@@ -1115,7 +1115,8 @@
 AC_MSG_CHECKING([optional compiler flags])
 CXX_FLAG_CHECK(NO_VARIADIC_MACROS, [-Wno-variadic-macros])
 CXX_FLAG_CHECK(NO_MISSING_FIELD_INITIALIZERS, [-Wno-missing-field-initializers])
-AC_MSG_RESULT([$NO_VARIADIC_MACROS $NO_MISSING_FIELD_INITIALIZERS])
+CXX_FLAG_CHECK(COVERED_SWITCH_DEFAULT, [-Wcovered-switch-default])
+AC_MSG_RESULT([$NO_VARIADIC_MACROS $NO_MISSING_FIELD_INITIALIZERS $COVERED_SWITCH_DEFAULT])
 
 dnl===-----------------------------------------------------------------------===
 dnl===