Refactor to use C++1y 'auto' semantics directly in lambdas with no specified
return type in C++1y mode. No functionality change intended. Extracted and
tweaked from a patch by Faisal Vali!


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@191354 91177308-0d34-0410-b5e6-96231b3b80d8