[preset] Test sourcekit-lsp in llbuild PR tests

To prevent failures in downstream projects, we need to test all the
projects that import llbuild (via swiftpm). Right now, that is
sourcekit-lsp.

rdar://54417572

Author: benlangmuir

utils/build-presets.ini

@@ -1483,47 +1483,28 @@ skip-test-llbuild
 #===------------------------------------------------------------------------===#
 
 [preset: buildbot_llbuild_macos_platform,tools=RA,stdlib=RA]
-mixin-preset=buildbot_incremental_base
-
-build-subdir=buildbot_incremental
+mixin-preset=mixin_swiftpm_macos_platform
 
 # Build Release without debug info, because it is faster to build.
 release
 assertions
 
-# Build llbuild & swiftpm here
-llbuild
-swiftpm
-
-skip-test-swift
-skip-test-cmark
+# Downstream projects that import llbuild+SwiftPM.
+sourcekit-lsp
 
 #===------------------------------------------------------------------------===#
 # Test llbuild on Linux builder
 #===------------------------------------------------------------------------===#
 
 [preset: buildbot_llbuild_linux_platform,tools=RA,stdlib=RA]
-mixin-preset=buildbot_incremental_base
-
-build-subdir=buildbot_incremental
+mixin-preset=mixin_swiftpm_linux_platform
 
 # Build Release without debug info, because it is faster to build.
 release
 assertions
 
-# Build llbuild & swiftpm here
-llbuild
-swiftpm
-
-xctest
-foundation
-llbuild
-
-skip-test-swift
-skip-test-cmark
-skip-test-foundation
-skip-test-libdispatch
-skip-test-xctest
+# Downstream projects that import llbuild+SwiftPM.
+sourcekit-lsp
 
 #===------------------------------------------------------------------------===#
 # Test SourceKit-LSP