Disallow assignments to calls with parentheses

[HertzDevil]

Resolves part of #14482 (comment). The other part about named arguments is unaffected.

foo = 1
foo() = 2     # Error: unexpected token: "="
foo.[]() = 3  # Error: unexpected token: "="
foo() += 4    # Error: unexpected token: "+="
foo.[]() += 5 # Error: unexpected token: "+="

By the way, foo = 1; foo() += 2 used to raise an internal NilAssertionError in the normalizer.