Introduces a new concept for binding results to matchers

as per Chandler's request:
- introduces a new matcher base type BindableMatcher that
  provides the bind() call
- makes all dynamic-cast matcher creation functions return
  BindableMatchers; the special case about dynamic-cast
  matchers is that the node they match on and the node
  their child matchers match on are the same node, just
  casted to a different type; thus, there is no ambiguity
  on what bind() matches on; additionally, those are the
  matchers that we name with nouns in the matcher language,
  so it's easy for users to intuitively know which matchers
  are bindable

To make this change possible, we got rid of a non-orthogonal
implementation of thisPointerType, which had an implicit
dynamic-cast matcher from CallExpr to CXXMemberCallExpr; as
alternative, we now provide a memberCall dynamic-cast matcher
and thisPointerType is a predicate on CXXMemberCallExpr.

Last, the ArgumentAdaptingMatcher is actually not required
for the implementation of makeDynCastAllOfComposite - this
simplification makes it more obvious where the bind() call
can be used based on the matcher creation function types.

llvm-svn: 160673