Bitcode: Add SimpleBitstreamCursor::getPointerToByte, etc.

Add API to SimpleBitstreamCursor to allow users to translate between
byte addresses and pointers.

  - jumpToPointer: move the bit position to a particular pointer.
  - getPointerToByte: get the pointer for a particular byte.
  - getPointerToBit: get the pointer for the byte of the current bit.
  - getCurrentByteNo: convenience function for assertions and tests.

Mainly adds unit tests (getPointerToBit/Byte already has a use), but
also preparation for eventually using jumpToPointer.

llvm-svn: 264546