Disable Linux support in Apple B&I builds

Swift SVN r25633

lib/Driver/Driver.cpp

@@ -1849,9 +1849,11 @@ const ToolChain *Driver::getToolChain(const ArgList &Args,
     case llvm::Triple::IOS:
       TC = new toolchains::Darwin(*this, Target);
       break;
+#if defined(SWIFT_ENABLE_TARGET_LINUX)
     case llvm::Triple::Linux:
       TC = new toolchains::Linux(*this, Target);
       break;
+#endif // SWIFT_ENABLE_TARGET_LINUX
     default:
       TC = nullptr;
     }