[nomaster] typecheck(q"class C") no longer crashes

MemberDefs alone can't be typechecked as is, because namer only names
contents of PackageDefs, Templates and Blocks. And, if not named, a tree
can't be typed.

This commit solves this problem by wrapping typecheckees in a trivial block
and then unwrapping the result when it returns back from the typechecker.

(cherry picked from commit 609047ba372ceaf06916d3361954bc949a6906ee)
Merge pull request #3551 from xeno-by/topic/typecheck-member-def

[nomaster] backports 609047ba37