[Clang] [Sema] Diagnose returning an initializer list from a lambda with a return type of void (#188904)
We previously didn’t diagnose attempts to return an initializer list from
a lambda with an explicit return type of `void`. This patch fixes that. It also
cleans up the error message so it actually says "lambda" instead of "block".
Fixes #188661
Signed off by: Kartik