[test] Enable LeakSanitizer on 64-bit Darwin ASan llvm builds

[fjricci]

Summary

Also disables leak checking on lto tests, due to many leaks reported
in the system's ld64.

[vsk]

LGTM, thanks!

[fjricci]

Updating this because the lit setup changed

test/lit.cfg

 
 # Static libraries are not built if BUILD_SHARED_LIBS is ON.
 if not config.build_shared_libs:
     config.available_features.add("static-libs")
 
 # Sanitizers.
 if 'Address' in config.llvm_use_sanitizer:
     config.available_features.add("asan")
+    # LeakSanitizer is disabled by default on Darwin, enable it on 64-bit builds
+    if re.match(r'^x86_64-apple.*', config.target_triple):
+        config.environment['ASAN_OPTIONS'] = 'detect_leaks=1'
 else:
     config.available_features.add("not_asan")
 if 'Memory' in config.llvm_use_sanitizer:
     config.available_features.add("msan")
 else:
     config.available_features.add("not_msan")
 if 'Undefined' in config.llvm_use_sanitizer:
     config.available_features.add("ubsan")

test/tools/lto/lit.local.cfg

 if not ('ld64_plugin' in config.available_features and 'X86' in config.root.targets):
     config.unsupported = True
+
+# These tests invoke ld64 from the system, which is not leak-free
+if "Address" in config.llvm_use_sanitizer:
+    config.environment['ASAN_OPTIONS'] = 'detect_leaks=0'