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;
  };
};