REFACTOR: Have PushLambdaScope return the LambdaScopeInfo that it creates.

No Functionality change.

This refactoring avoids having to call getCurLambda right after PushLambdaScope, to obtain the LambdaScopeInfo that was created during the call to PushLambdaScope. 

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@194438 91177308-0d34-0410-b5e6-96231b3b80d8