Support: Functions for consuming endian specific data from a buffer.

This adds a function to Endian.h that reads from and updates a pointer
into a buffer with endian specific data. This is more convenient for
stream-like reading of data than endian::read.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@204693 91177308-0d34-0410-b5e6-96231b3b80d8