<rdar://problem/10568905>

Fixed an issue where our new accelerator tables could cause a crash
when we got a full 32 bit hash match, yet a C string mismatch.

We had a member variable in DWARFMappedHash::Prologue named 
"min_hash_data_byte_size" the would compute the byte size of HashData
so we could skip hash data efficiently. It started out with a byte size
value of 4. When we read the table in from disk, we would clear the
atom array and read it from disk, and the byte size would still be set
to 4. We would then, as we read each atom from disk, increment this count. 
So the byte size of the HashData was off, which means when we get a lookup
whose 32 bit hash does matches, but the C string does NOT match (which is
very very rare), then we try and skip the data for that hash and we would
add an incorrect offset and get off in our parsing of the hash data and 
cause this crash. 

To fix this I added a few safeguards:
1 - I now correctly clear the hash data size when we reset the atom array using the new DWARFMappedHash::Prologue::ClearAtoms() function. 
2 - I now correctly always let the AppendAtom() calculate the byte size of the hash (before we were doing things manually some times, which was correct, but not good)
3 - I also track if the size of each HashData is a fixed byte size or not, and "do the right thing" when we need to skip the data.
4 - If we do get off in the weeds, then I make sure to return an error and stop any further parsing from happening. 

llvm-svn: 147334