Sema: Do not allow lambda expressions to appear inside of constant expressions

We would previously not diagnose this which would lead to crashes (on
very strange code).

This fixes PR17675.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@193397 91177308-0d34-0410-b5e6-96231b3b80d8
5 files changed