Rewrote the bitfield logic. Major changes include:

- Removed the BitfieldMap class because it is unnecessary.
  We now just track the most recently added field.

- Moved the code that calculates bitfield widths so it
  can also be used to determine whether it's necessary
  to insert anonymous fields.

- Simplified the anonymous field calculation code into
  three cases (two of which are resolved identically).

- Beefed up the bitfield testcase.

llvm-svn: 169449