Fix the underlying problem that was causing read(0) to be called: sometimes the

bitcode writer would generate abbrev records saying that the abbrev should be
filled with fixed zero-bit bitfields (this happens in the .bc writer when 
the number of types used in a module is exactly one, since log2(1) == 0).

In this case, just handle it as a literal zero.  We can't "just fix" the writer
without breaking compatibility with existing bc files, so have the abbrev reader
do the substitution.

Strengthen the assert in read to reject reads of zero bits so we catch such 
crimes in the future, and remove the special case designed to handle this.

llvm-svn: 174801