[PDB] Fix unsigned integer overflow

When building with -fsanitize=unsigned-integer-overflow, this code
causes a diagnostic like:

    ../../llvm/lib/DebugInfo/PDB/Native/GSIStreamBuilder.cpp:159:15:
    runtime error: unsigned integer overflow:
    90 - 229 cannot be represented in type 'unsigned long'

unsigned integer overflow is well defined and it isn't an issue in
practice, but in obscure scenarios (S1.size() small, S2.size over 2GB
on 32-bit systems) it could even be a bug.

So use the usual idiom for implementing cmp functions instead of the
gernally considered buggy idiom :)
See e.g. https://www.gnu.org/software/libc/manual/html_node/Comparison-Functions.html
or https://stackoverflow.com/questions/10996418/efficient-integer-compare-function/10997428#10997428

Differential Revision: https://reviews.llvm.org/D97557