Begin adding experimental support for preserving use-list ordering of bitcode

files.  First, add a new block USELIST_BLOCK to the bitcode format.  This is 
where USELIST_CODE_ENTRYs will be stored.  The format of the USELIST_CODE_ENTRYs
have not yet been defined.  Add support in the BitcodeReader for parsing the
USELIST_BLOCK.
Part of rdar://9860654 and PR5680.

llvm-svn: 146078