Avoid lambdas in default member initializers to work around clang bug

On Windows, Clang is mangling lambdas in default member initializers
incorrectly. See PR31197.

This is causing redness on the self-host bots. Work around the problem
locally so we aren't blind to further issues.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@288089 91177308-0d34-0410-b5e6-96231b3b80d8