Very basic property getter support

Property decorators were ignored except for code-completion, trying to
access properties would get a function rather than the returned type.

CCBUG: 372273

Just replace property functions with their return type in
ExpressionVisitor::visitCall().

This only works for getter-only properties - if there's a setter function
for the property (which idiomatically has the same name), visitCall() finds
that declaration instead and gives the wrong return type.

Still, it's useful in a good proportion of cases.

To make this work properly, I think we need a specific declaration type
for properties, and to update that when visiting the getter/setter
instead of creating the function declarations directly in the class?

One new test fail->pass.
Two new tests still fail.
No test regressions.

Differential Revision: https://phabricator.kde.org/D4207