[PDB] Fix hash function used to write /src/headerblock
lld-link used to write PDB files that DIA couldn't recover natvis
files from if:
- The global strings table was > 64kiB
- There were at least 3 natvis files
The cause was that the hash function for the /src/headerblock stream
was incorrect: It needs to be truncated to 16 bit.
If the global strings table was <= 64kiB, truncating to 16 bit is a
no-op, so this wasn't needed for small programs.
If there are only 1 or 2 natvis files, then the growth strategy in
HashTable::grow() would mean the hash table would have 2 buckets (for 1
natvis file) or 4 buckets (for 4 natvis files), and since the hash
function is used modulo number of buckets, and since 2 and 4 divide
0x10000, the missing % 0x10000 is a no-op there too. For 3 natvis
files, the hash table grows to 6 buckets, which has a factor that's not
common with 0x10000 and the difference starts to matter.
Differential Revision: https://reviews.llvm.org/D61277