The filter name of a function should match the spelling of the function
name according to the language except that we don't insert _ for unnamed
parameters because underscores are legal inside identifiers and we don't
want to introduce spurious matches (also, we keep the () on a
parameterless function for disambiguation).
rdar://problem/26118915
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.
