[ORC] Add unit test for MemoryFlags APIs, don't dereference end() iterator.

In AllocGroupSmallMap::find(AllocGroup) we were calling lower_bound(...) and
then unconditionally dereferencing the result

[ORC] Add unit test for MemoryFlags APIs, don't dereference end() iterator.

In AllocGroupSmallMap::find(AllocGroup) we were calling lower_bound(...) and
then unconditionally dereferencing the resulting iterator, however
lower_bound(...) may return end() if the value being searched for is higher
than any value present in the map. This patch adds a check for end() before
the dereference to guard against dereference of end().

This commit also adds some basic unit tests for MemProt and AllocGroupSmallMap.

rdar://129662981

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