Cleaned up results formatter options hand-off.

* --results-formatter-options renamed to --results-formatter-option,
  with short version of -O

* Multiple --results-formatter-option=OPTION can be specified.  The
  comma-separating mechanism has been removed.

* XunitFormatter options modified: -n and -r are now short forms of
  --ignore-skip-name and --ignore-skip-reason.  Those long option
  names were tweaked lightly.  They also can be specified multiple
  times on the command line.  The comma-separating, multiple-pattern-
  per-option mechanism has been removed.

One can now specify:

  dotest.py --results-file stdout -O-ndsym -O-nlldb-mi

for example, to ignore reporting skips for dsym-related or lldb-mi-related
tests in the xUnit report.

llvm-svn: 248384