Switch the bitcode reader interface to take a MemoryBuffer instead of knowing

Switch the bitcode reader interface to take a MemoryBuffer instead of knowing
anything about disk I/O itself.  This greatly simplifies its interface -
eliminating the need for the ReaderWrappers.cpp file.

This adds a new option to llvm-dis (-bitcode) which instructs it to read
the input file as bitcode.  Until/unless the bytecode reader is taught to
read from MemoryBuffer, there is no way to handle stdin reading without it.

I don't plan to switch the bytecode reader over, I'd rather delete it :),
so the option will stay around temporarily.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@36554 91177308-0d34-0410-b5e6-96231b3b80d8