[cxx2a] P0624R2 fix: only lambdas with no lambda-capture are default-constructible and assignable.

This is a fix for rG864949 which only disabled default construction and
assignment for lambdas with capture-defaults, where the C++2a draft
disables them for lambdas with any lambda-capture at all.

Patch by Logan Smith!

Differential Revision: https://reviews.llvm.org/D64058

llvm-svn: 365406