[IR] Limit bits used for CallingConv::ID, update tests

[IR] Limit bits used for CallingConv::ID, update tests

Use 10 bits to represent calling convention ID's instead of 13, and
update the bitcode compatibility tests accordingly. We now error-out in
the bitcode reader when we see bad calling conv ID's.

Thanks to rnk and dexonsmith for feedback!

Differential Revision: http://reviews.llvm.org/D13826

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@251452 91177308-0d34-0410-b5e6-96231b3b80d8