[PDB] Fix a bug where we were serializing hash tables incorrectly.

There was some code that tried to calculate the number of 4-byte
words required to hold N bits, but it was instead computing the
number of bytes required to hold N bits.  This was leading to
extraneous data being output into the hash table, which would
cause certain operations in DIA (the Microsoft PDB reader) to
fail.

llvm-svn: 327675