First, remove lit configuration that sets ASAN_OPTIONS to detect_leaks=1
because this is already the default when leak detection is supported.
This removes a bit of duplication between various lit.cfg files.
Second, add a new feature 'leak-detection' if we're targetting x86_64
(not i386) on Linux.
Third, change a couple of tests that need leak detection to require the
new 'leak-detection' feature.