Add GetInteriorSymbols API for IMethodSymbols

[jmarolf]

There is no good way to get interior symbols on a method symbol today. Its currently impossible for completion to do it performantly with out a speculative SemanticModel

Notes: #11193

[rchande]

Worked around in #11398

[gafter]

By design, the interior symbols of a symbol are only stable within a particular SemanticModel. So this would have to be an API on the SemanticModel of the enclosing symbol.

It is not just for method symbols. Even a field's initializer can declare locals (out variables or pattern variables).

[CyrusNajmabadi]

Not sure what the driving scenario is for this. Even if the compiler added this, i'm not sure where we'd use this or how we'd measure that it benefitted us. Closing out.