[ADT] Improve the genericity of llvm::enumerate().

There were some issues in the implementation of enumerate()
preventing it from being used in various contexts.  These were
all related to the fact that it did not supporter llvm's
iterator_facade_base class.  So this patch adds support for that
and additionally exposes a new helper method to_vector() that
will evaluate an entire range and store the results in a
vector.

Differential Revision: https://reviews.llvm.org/D30853

llvm-svn: 297633