Address a few of the issues in GCDAProfiling I noted when looking

through Bill's patch:

1) Correctly test the file descriptor after the sceond attempt at
   creating the file.
2) Make the filename a global so that we can issue error messages from
   other routines.
3) Check errno in several places and print it out so that errors are
   easier to track down.

I don't really expect any of these to fix the current failures I'm
seeing, but I'm hopeful they'll at least let me debug them.

llvm-svn: 184799