Add a new BitstreamEntry concept, and add two helper methods for walking

Add a new BitstreamEntry concept, and add two helper methods for walking
through a BitstreamCursor that produce it: advance() and
advanceSkippingSubblocks(), representing the two most common ways clients
want to walk through bitcode.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@172919 91177308-0d34-0410-b5e6-96231b3b80d8