[PDB] One more fix for hasing GSI records.

The reference implementation uses a case-insensitive string
comparison for strings of equal length.  This will cause the
string "tEo" to compare less than "VUo".  However we were using
a case sensitive comparison, which would generate the opposite
outcome.  Switch to a case insensitive comparison.  Also, when
one of the strings contains non-ascii characters, fallback to
a straight memcmp.

The only way to really test this is with a DIA test.  Before this
patch, the test will fail (but succeed if link.exe is used instead
of lld-link).  After the patch, it succeeds even with lld-link.

llvm-svn: 336464