[turbofan] Make FieldAccess::name usable in practice.

[turbofan] Make FieldAccess::name usable in practice.

This debugging only field was mostly unpopulated because it required
names to be allocated in the heap. Changing it to a raw C-string allows
passing sensible values without having access to the heap.

R=bmeurer@chromium.org

[Michael Starzinger, 2015-12-10 14:53:12]

As per offline discussion: I learned that we want to potentially use the
{FieldAccess::name} value for alias analysis. Then it makes sense to keep it
Handle<Name> type. But we need to make sure it is always properly populated. I
will rework this CL accordingly.