Tweak the LoopDistribute debug output to be prefixed with "LDist: ", get
it to be stable, and extend update_analyze_test_checks.py trivially to
support this output.
Introduce a Loop::getLocStr stolen from LoopVectorize's static function
getDebugLocString in order to have uniform debug output headers across
LoopVectorize, LoopAccessAnalysis, and LoopDistribute. The motivation
for this change is to have UpdateTestChecks recognize the headers and
automatically generate CHECK lines for debug output, with minimal
special-casing.