[Profiler] Fix handling of property wrapper backing inits
Previously we were walking into the
PropertyWrapperValuePlaceholderExpr when generating
coverage for a property wrapper backing initializer.
This meant that we were duplicating the coverage of
the initializer expression, and it could cause
crashes if a refined counter was introduced within
the top-most expression region, such as with a
throwing expression in a `try!`.
rdar://118939162
