Stop parsing frontend arguments directly and use the driver instead. The
most intersting part of this change is that it forces us to consider
whether our compiler invocation will have inputs or not. We have
several kinds of requests that need to create a compiler instance, but
not parse any inputs (interface-generation, doc-info, and indexing when
operating on a module instead of source files).
Incidentally, add an error when trying to do doc-info on multiple source
files. This was already very broken (assertion failures and bogus source
locations), so add an error for it.
rdar://problem/17897287
The code goes into its own sub-tree under 'tools' but tests go under 'test',
so that running 'check-swift' will also run all the SourceKit tests.
SourceKit is disabled on non-darwin platforms.
