The attached patch adds basic support for handling strnlen
as a built-in function.  It touches the strlen pass where
it folds constant results of the function, and builtins.c
to add simple support for expanding strnlen calls with known
results.  It also changes calls.c to detect excessive bounds
to the function and unsafe calls with arguments declared
attribute nonstring.

A side-effect of the strlen change I should call out is that
strlen() calls to all zero-length arrays that aren't considered
flexible array members (i.e., internal members or non-members)
are folded into zero.  No warning is issued for such invalid
uses of zero-length arrays but based on the responses to my
question Re: aliasing between internal zero-length-arrays and
other members(*) it sounds like one would be appropriate.
I will see about adding one in a separate patch.

Martin

[*] https://gcc.gnu.org/ml/gcc/2018-06/msg00046.html