Replace qmake variable BLACK_CONFIG with a system based on json files.

The config is loaded from json files by qmake, and we define the qmake
function `swiftConfig` for checking whether a feature is enabled.
This function can be directly used in `buildconfig_gen.cpp.in`,
so the trick with C++ comment tokens in variables is not needed.

src/plugins/simulator/fscommon/fscommon.pro

@@ -17,7 +17,7 @@ HEADERS += *.h
 LIBS *= -lsimulatorplugincommon
 addStaticLibraryDependency(simulatorplugincommon)
 
-contains(BLACK_CONFIG, FSUIPC) {
+swiftConfig(sims.fsuipc) {
     equals(WORD_SIZE,32) {
         DEFINES += SWIFT_USING_FSUIPC32
     }