[build-script] Add an incremental preset for Xcode.

Author: gottesmm

utils/build-presets.ini

@@ -247,6 +247,12 @@ skip-test-ios
 skip-test-tvos
 skip-test-watchos
 
+[preset: buildbot_incremental,tools=RA,stdlib=RA,xcode]
+mixin-preset=buildbot_incremental,tools=RA,stdlib=RA
+build-subdir=buildbot_incremental-xcode
+
+xcode
+dash-dash
 
 # This preset is used by CI to test swift-corelibs-xctest.
 [preset: buildbot_incremental,tools=RA,stdlib=RA,XCTest]