[PDB] Sort globals symbols by name in GSI hash buckets.

It seems like the debugger first computes a symbol's bucket,
and then does a binary search of entries in the bucket using the
symbol's name in order to find it.  If the bucket entries are not
in sorted order, this obviously won't work.  After this patch a
couple of simple test cases show that we generate an exactly
identical GSI hash stream, which is very nice.

llvm-svn: 336405