Initial cleanups of the param-attribute code in the bitcode reader/writer.

Rename the PARAMATTR_CODE_ENTRY to PARAMATTR_CODE_ENTRY_OLD. It will be replaced
by another encoding. Keep around the current LLVM attribute encoder/decoder
code, but move it to the bitcode directories so that no one's tempted to use
them.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@174335 91177308-0d34-0410-b5e6-96231b3b80d8