Add fix-it notes to the nullability consistency warning.

This is especially important for arrays, since no one knows the proper
syntax for putting qualifiers in arrays.

    nullability.h:3:26: warning: array parameter is missing a nullability type specifier (_Nonnull, _Nullable, or _Null_unspecified)
    void arrayParameter(int x[]);
                             ^
    nullability.h:3:26: note: insert '_Nullable' if the array parameter may be null
    void arrayParameter(int x[]);
                             ^
                              _Nullable
    nullability.h:3:26: note: insert '_Nonnull' if the array parameter should never be null
    void arrayParameter(int x[]);
                             ^
                              _Nonnull

rdar://problem/29524992

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@290132 91177308-0d34-0410-b5e6-96231b3b80d8