add some simple designator testcases. Reject things like this:

add some simple designator testcases.  Reject things like this:

struct foo Y[10] = {
  [4] .arr [2] 4  // expected-error {{expected '=' or another designator}}
};

because the "missing equals" extension only is valid if there
is exactly one array designator.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@58215 91177308-0d34-0410-b5e6-96231b3b80d8