[cxx1z-constexpr-lambda] Make a lambda's closure type eligible as a literal-type...

[cxx1z-constexpr-lambda] Make a lambda's closure type eligible as a literal-type in C++1z

Additionally, for pre-C++1z, instead of forbidding a lambda's closure type from being a literal type through circumlocutorily setting HasNonLiteralTypeFieldsOrBases falsely to true -- handle lambda's more directly in CXXRecordDecl::isLiteral().

One additional small step towards implementing constexpr-lambdas.

Thanks to Richard Smith for his review!
https://reviews.llvm.org/D22662

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@276514 91177308-0d34-0410-b5e6-96231b3b80d8