InstrProf: Read unsigned numbers with strtoul and strtoull

Fixes a bug where unsigned numbers are read using strtol and strtoll.

I don't have a testcase because this bug is effectively unobservable
right now.  To expose the problem in the hash, we would need a function
with greater than INT64_MAX counters, which we don't handle anyway.  To
expose the problem in the function count, we'd need a function with
greater than INT32_MAX counters; this is theoretically observable, but
it isn't a practical testcase to check in.

An upcoming commit changes the hash to be non-trivial, so we'll get some
coverage eventually.

<rdar://problem/16435801>

llvm-svn: 206001