[build-script] Add a new LLVM only incremental linux preset.

Author: gottesmm

utils/build-presets.ini

@@ -688,6 +688,15 @@ validation-test=0
 long-test=1
 test-optimized=0
 
+[preset: buildbot_incremental_linux,llvm-only]
+mixin-preset=buildbot_incremental_linux
+build-subdir=buildbot_incremental_linux_llvmonly
+
+dash-dash
+
+skip-build-swift
+skip-build-cmark
+
 
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 # OS X Package Builders