Implements the convenience matcher findAll.

We found that findAll has been implemented incorrectly multiple times
by various people using the matchers. To prevent further wasted
development effort, it makes sense to add it as convenience matcher
implemented as eachOf(m, forEachDescendant(m)).

This patch also updates the docs with the new matchers.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@174320 91177308-0d34-0410-b5e6-96231b3b80d8