[Sema] Replace mem_fn with lambdas. NFC.

I'm told that some optimizers like lambdas a lot more than mem_fn.
Given that the readability difference is basically nil, and we seem to
use lambdas basically everywhere else, it seems sensible to just use
lambdas.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@276577 91177308-0d34-0410-b5e6-96231b3b80d8