Correct logic for constructing symbol file names

The path to the symbol file name is:
  $DEBUG_FILE_NAME/$BUILD_ID/$SYMFILE_NAME

Where $DEBUG_FILE_NAME is the name of the file from
which the debug info was originally extracted - the PDB
on Windows or the executable / shared library directly
on other platforms.

$SYMFILE_NAME is the same as $DEBUG_FILE_NAME minus the '.pdb'
suffix, if it has one.