Mark lambda closure classes as being implicitly-generated.

Summary: Closure classes for C++ lambdas are always compiler-generated.  This one-line change calls setImplicit(true) on them at creation time, such that a default RecursiveASTVisitor (or any for which shouldVisitImplicitCode returns false) will skip them.

Reviewers: rsmith, dblaikie

Reviewed By: dblaikie

CC: klimek, revane, cfe-commits, jordan_rose

Differential Revision: http://llvm-reviews.chandlerc.com/D1593

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@190073 91177308-0d34-0410-b5e6-96231b3b80d8