[llvm][Instrumentor] Fix non-determinism in Instrumentor (#198833)
In InstrumentorStubPrinter the StringMap was being iterated over, which
broke the Instrumentation/Instrumentor/write_config.ll test under
LLVM_REVERSE_ITERATION builds. We can use a MapVector to ensure the
order is stable. We only need to update the test json ordering to match
the stable order for with and without LLVM_REVERSE_ITERATION.