Parser: Array decls with static but without array size are illformed

Array declarators involving the static keyword take on two forms:
    D[ static type-qualifier-listopt assignment-expression ]
    D[ type-qualifier-list static assignment-expression ]

Raise a diagnostic if the assignment-expression is missing.

This fixes PR20584.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@215187 91177308-0d34-0410-b5e6-96231b3b80d8