Add -Wno-nested-anon-types to -pedantic builds of LLVM. This Clang warning
catches uses of an extremely minor and widely-available C++ extension (which
every C++ compiler I could find supports, but EDG and Clang reject in strict
mode).

The diagnosed code pattern looks like this:

struct X {
  union {
    struct {
      int a;
      int b;
    } S;
  };
};


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@174103 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/autoconf/configure.ac b/autoconf/configure.ac
index 21c3eeb..0a291c1 100644
--- a/autoconf/configure.ac
+++ b/autoconf/configure.ac
@@ -1264,6 +1264,7 @@
 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])
+CXX_FLAG_CHECK(NO_NESTED_ANON_TYPES, [-Wno-nested-anon-types])
 CXX_FLAG_CHECK(COVERED_SWITCH_DEFAULT, [-Wcovered-switch-default])
 dnl GCC's potential uninitialized use analysis is weak and presents lots of
 dnl false positives, so disable it.
@@ -1299,7 +1300,7 @@
   NO_UNINITIALIZED=
   NO_MAYBE_UNINITIALIZED=
 fi
-AC_MSG_RESULT([$NO_VARIADIC_MACROS $NO_MISSING_FIELD_INITIALIZERS $COVERED_SWITCH_DEFAULT $NO_UNINITIALIZED $NO_MAYBE_UNINITIALIZED])
+AC_MSG_RESULT([$NO_VARIADIC_MACROS $NO_MISSING_FIELD_INITIALIZERS $NO_NESTED_ANON_TYPES $COVERED_SWITCH_DEFAULT $NO_UNINITIALIZED $NO_MAYBE_UNINITIALIZED])
 
 AC_ARG_WITH([python],
             [AS_HELP_STRING([--with-python], [path to python])],