Check that the profile runtime works as expected. This tests the
functions that are meant to be available to advanced users.
In particular, check that the `atexit()` hook can be disabled by
defining a custom `__llvm_profile_runtime` variable, that the libc
dependencies are optional, and that the various functions for writing
out files work for basic cases.
llvm-svn: 208460
Duncan P. N. Exon Smith