[leak-checker] Make sure to actually enable the leak checker when compiling with the leak checker presets.

Fixes refactoring error in 14c0a3e...

*shakes head*.

Author: gottesmm

utils/build-presets.ini

@@ -520,6 +520,7 @@ build-ninja
 build-swift-static-stdlib=0
 build-swift-stdlib-unittest-extra=1
 compiler-vendor=apple
+swift-runtime-enable-leak-checker=1
 
 [preset: buildbot_incremental_leaks,tools=RA,stdlib=R]
 mixin-preset=buildbot_incremental_leaks