'ParserUnit' is used for analyzing syntax structures _mainly_ in
SourceKit.
Since we removed IfConfigDecl from AST, ParserUnit didn't
inclue any AST in #if ... #endif regions even for active region because
it used to consider all inactive. Instead, consider every region
"active" and include all the AST nodes.
rdar://117387631
The SourceKit code formatting tools have effectively been replaced
by swift-syntax's basic formatting functionality and more full-featured
formatters like swift-format and SwiftFormat. Remove the special
handling of IfConfigDecl as staging for the removal of IfConfigDecl.
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.
