Fix a bug in the conversion to ErrorOr.

The regular end of the bitcode parsing is in the  BitstreamEntry::EndBlock
case.

Should fix the LTO bootstrap on OS X (this function is only used by ld64).

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@212357 91177308-0d34-0410-b5e6-96231b3b80d8