[Sema] Improve side effect checking for unused-lambda-capture warning

Summary:
Don't warn about unused lambda captures that involve copying a
value of a type that cannot be trivially copied and destroyed.

Fixes PR31977

Reviewers: rsmith, aaron.ballman

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D30327

llvm-svn: 296602