Disallow weird assignments

[FnControlOption]

Closes #12830

Added Call#dot_location so that the AST for a[0] can be differentiated from a.[] 0. This makes it easier to disallow a.[] 0 = 1

Added a check for whether Call#named_args is nil so that a b: 0 = 1 is disallowed and not treated like an assignment to a (where b: 0 was ignored)

[FnControlOption]

Found some more edge cases:

foo = [1, 2, 3]
#foo.[]? = 1         # Error: undefined method '[]?='
#foo.[]? += 1        # Error: wrong number of arguments
#foo.[0]? = 1        # Error: unexpected token: "="
#foo.[0]? += 1       # Error: unexpected token: "+="
#foo[0]? = 1         # Error: unexpected token: "="
#foo[0]? += 1        # Error: unexpected token: "+="
#foo.tap &.[0]? = 1  # Error: undefined method '[]?='
#foo.tap &.[] 0 = 1  # Error: no overload matches
#foo.tap &.[] = 1    # Error: no overload matches

class Foo
  def []=(value : Int)
    puts value
  end
end
foo = Foo.new
#foo.tap &.[]? = 1   # Error: undefined method '[]?='
#foo.tap &.[0]? = 1  # Error: undefined method '[]?='
foo.tap(&.[]=(1))    # 1
foo.tap &.[] = 1     # 1
foo.tap &.[]=(1)     # 1
foo.tap &.[] 0 = 1   # 1
foo.tap &.[](0) = 1  # 1

class Foo
  def foo
    Foo.new
  end
  def []=(index : Int, value : Int)
    puts [index, value]
  end
end
foo.tap &.foo[0] = 1    # [0, 1]
foo.tap &.foo.[] 0 = 1  # [0, 1]
#foo.tap &.foo[0]? = 1  # Error: unexpected token: "="

Maybe instead of dot_location, a field named args_in_brackets could be added