The `-force-single-frontend-invocation` flag predates WMO and is now an
alias for `-whole-module-optimization`. We should use the latter and let
the former fade into history.
This change PCMacro and PlaygroundTransform to return an a moduleID and
fileID in addition to the source location information. The Frontend has
been changed to run PCMacro and PlaygroundTransform on all input files
instead of the main file only.
The tests have been updated to conform to these changes with an addition
of module and file ID specific tests. The Playgrounds related tests were
adjusted to make a module out of the stub interface files since those
files should not have PCMacro and PlaygroundTransform applied to them.
rdar://problem/50821146
