ObjCXX: fix a crash during typo correction.

For ObjCXX, we can create a CastExpr with Kind being CK_UserDefinedConversion
and SubExpr being BlockExpr. Specifically one can return BlockExpr from
BuildCXXMemberCallExpr and the result can be used to build a CastExpr.

Fix the assumption in CastExpr::getSubExprAsWritten that SubExpr can only
be CXXMemberCallExpr.

rdar://problem/24364077


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@259591 91177308-0d34-0410-b5e6-96231b3b80d8