[XRay][clang] Support capturing the implicit `this` argument to C++ class member functions

Summary:
Before this change, we couldn't capture the `this` pointer that's
implicitly the first argument of class member functions. There are some
interesting things we can do with capturing even just this single
argument for zero-argument member functions.

Reviewers: rnk, pelikan

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D34052

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@305544 91177308-0d34-0410-b5e6-96231b3b80d8