[llvm] Fix some test failures with EXPENSIVE_CHECKS and libstdc++

DebugLocEntry assumes that it either contains 1 item that has no fragment
or many items that all have fragments (see the assert in addValues).

When EXPENSIVE_CHECKS is enabled, _GLIBCXX_DEBUG is defined. On a few machines
I've checked, this causes std::sort to call the comparator even
if there is only 1 item to sort. Perhaps to check that it is implemented
properly ordering wise, I didn't find out exactly why.

operator< for a DbgValueLoc will crash if this happens because the
optional Fragment is empty.

Compiler/linker/optimisation level seems to make this happen
or not. So I've seen this happen on x86 Ubuntu but the buildbot
for release EXPENSIVE_CHECKS did not have this issue.

Add an explicit check whether we have 1 item.

Reviewed By: aprantl

Differential Revision: https://reviews.llvm.org/D130156