SourceKit invariantly expands the last argument in a function call as trailing closure,
if it is of function type. However, there are situations when inline closures
are preferred; for example, when the last argument is not the only closure in the function
call. This patch modifies SourceKit so that when the argument contains multiple closures,
the last argument is expanded as inline closure.
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.
