[build-script] Add buildbot_linux,without_host_swift preset

Based on a patch from Evan Wilde, introduce a new preset that disables
the features that require Swift host tools. This involves disabling
early swift-syntax (used in the compiler for macros et al) and early
swift-driver.

Author: DougGregor

utils/build-presets.ini

@@ -930,6 +930,13 @@ mixin-preset=
 
 skip-test-swiftdocc
 
+[preset: buildbot_linux,without_host_swift]
+mixin-preset=
+  buildbot_linux
+
+skip-early-swiftsyntax
+skip-early-swift-driver
+
 [preset: buildbot_linux_crosscompile_wasm]
 mixin-preset=buildbot_linux