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