[Build] Limit Ubuntu 18.04 LSAN CI to 12 link jobs

It's currently running out of memory, likely due to the machine's high
core count and linking many jobs at once. Limit to 12 only.

(cherry picked from commit a705ee2)

Author: bnbarham

utils/build-presets.ini

@@ -1197,6 +1197,9 @@ reconfigure
 [preset: buildbot_incremental_linux,lsan,tools=RDA,stdlib=DA,test=no]
 build-subdir=buildbot_incremental_lsan
 
+llvm-cmake-options=-DLLVM_PARALLEL_LINK_JOBS=12
+swift-cmake-options=-DSWIFT_PARALLEL_LINK_JOBS=12
+
 release-debuginfo
 assertions
 enable-lsan