profile: explicitly initialize file name in the test

The instrprofile-write-file-only test was failing on the builtbots.  The runtime
library initialization is explicitly being discarded to avoid the atexit hook.
However, this would also prevent the filename from being initialized.  Thus,
when the write file was invoked, the filename would not be setup, and the test
would fail as the profiling data would never be written out.  Explicitly
initialize the filename to ensure that the data is written out when requested.
This should hopefully finally get the build bots all green again.

llvm-svn: 209099