[ADT] "Inline" TestAndEraseFromSet into SetVector::remove_if (NFC) (#155790)
TestAndEraseFromSet is used only from SetVector::remove_if. This
patch "inlines" the struct into its sole user in the form of a lambda
function.
FWIW, "git blame" shows that TestAndEraseFromSet dates back to 2012.
Most likely, the lambda function wasn't an option yet back then.