Avoid use of lambda to work around a clang bug. (#1883)

Clang has a bug [1] in x86 Windows that is exposed by the use of lambdas with "unforwardable" prototypes. The error is "error: cannot compile this forwarded non-trivially copyable parameter yet", and the message was introduced in [2] (used to be an assertion).

[1] https://llvm.org/bugs/show_bug.cgi?id=28299
[2] https://github.com/microsoft/checkedc-clang/commit/feb1567e07573100ea14f9aea02f81463e791496