[llvm-profdata] Fix unhandled enumeration values warning

Fix this:

llvm/tools/llvm-profdata/llvm-profdata.cpp:78:15: warning: 12 enumeration values not handled in switch: 'success', 'eof', 'unrecognized_format'... [-Wswitch]
      switch (instrError) {
                    ^

llvm-svn: 253413