[ASTMatchers] Use provided target NodeKind instead of inferring it from the matchers.

[ASTMatchers] Use provided target NodeKind instead of inferring it from the matchers.

Individual matchers might not be convertible to each other's kind, but
they might still all be convertible to the target kind.
All the callers already know the target kind, so just pass it down.

llvm-svn: 242534