The SILGen testsuite consists of valid Swift code covering most language
features. We use these tests to verify that no unknown nodes are in the
file's libSyntax tree. That way we will (hopefully) catch any future
changes or additions to the language which are not implemented in
libSyntax.
emitLValue always has to occur in a writeback scope, even if the lvalue isn't formally accessed until later, because some lvalue productions immediately access their parent lvalue expression (namely optional chaining expressions). Fixes rdar://problem/26642478.
