If we have an identifier followed by either `[` or
a generic argument list, avoid turning it into a
binding pattern, as that would be invalid. This
is similar to the existing rule we have where a
following `(` prevents a binding pattern from
being formed.
This allows patterns such as `let E<Int>.foo(x)` and
`let (y[0], x)` to compile, where `x` is treated
as a binding, but no other identifier is.
rdar://108738034
