[lldb] Make SBType::FindDirectNestedType work with expression ASTs (#89183)

The types we get out of expressions will not have an associated symbol
file, so the current method of looking up the type will fail. Instead, I
plumb the query through the TypeSystem class. This correctly finds the
type in both cases (importing it into the expression AST if needed). I
haven't measured, but it should also be more efficient than doing a type
lookup (at least, after the type has already been found once).