intuitive.
It does NOT update the value if the key is already in the map,
it also returns false if the key is already in the map, regardless
if the value matched.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@83458 91177308-0d34-0410-b5e6-96231b3b80d8
127445818e