Make sure every call to synthesized functions is valid

[litherum]

Migrated from https://bugs.webkit.org/show_bug.cgi?id=189977:

At 2018-09-25T23:20:15Z, mmaxfield@apple.com wrote:
We synthesize things like operator.length() inside CallExpression._resolveByInstantiation(). We should make sure we catch things like

int x = 7;
x.length(123, 456, 789);