Bitcode: Fix reading and writing of ConstantDataVectors of halfs

In r254991 I allowed ConstantDataVectors to contain elements of
HalfTy, but I missed updating the bitcode reader and writer to handle
this, so now we crash if we try to emit bitcode on programs that have
constant vectors of half.

This fixes the issue and adds test coverage for reading and writing
constant sequences in bitcode.

llvm-svn: 256982