Add an eachOf matcher.

eachOf gives closure on the forEach and forEachDescendant matchers.
Before, it was impossible to implement a findAll matcher, as matching
the node or any of its descendants was not expressible (since anyOf
only triggers the first match).

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@174315 91177308-0d34-0410-b5e6-96231b3b80d8