[build-preset] Add preset buildbot_incremental,tools=RA,llvm-only

This preset just builds llvm via build-script. Its intention is to enable bots
to validate that we are properly configuring LLVM/Clang so that all LLVM/Clang
tests pass.

We have the unfortunate burden of attempting to match LLVM/Clang's build exactly
with build-script. This is a small step towards validating that by ensuring that
when we configure LLVM/Clang we can at least run their tests.

For expediency, I am just going to trigger the LLVM/Clang tests on the bots
without using build-script-impl since there are a bunch of large changes
occuring there.

rdar://26559922

Author: gottesmm

utils/build-presets.ini

@@ -419,6 +419,19 @@ dash-dash
 
 swift-enable-lto
 
+[preset: buildbot_incremental,tools=RA,llvm-only]
+build-subdir=buildbot_incremental_llvmonly
+
+# Build Release without debug info, because it is faster to build.
+release-debug
+assertions
+test=0
+
+dash-dash
+
+# Do not build swift or cmark
+skip-build-swift
+skip-build-cmark
 
 #===------------------------------------------------------------------------===#
 # A setting to run a buildbot that passes extra swift args when compiling