Prevent DCE on __lsan_is_turned_off and re-enable test case

[fjricci]

Summary

-dead_strip in ld64 strips weak interface symbols, which I believe
is most likely the cause of this test failure. Re-enable after marking the interface
function as used.

[kcc]

May I ask you to also update the comments around the definition of __lsan_is_turned_off in lsan_interface.h?

// To avoid dead stripping you may need to define this function with __attribute__((used))

[fjricci]

Good call, thanks.

[fjricci]

Add documentation

[kcc]

LGTM, thanks!

test/lsan/TestCases/link_turned_off.cc

 // Test for disabling LSan at link-time.
 // RUN: LSAN_BASE="use_stacks=0:use_registers=0"
 // RUN: %clangxx_lsan %s -o %t
 // RUN: %env_lsan_opts=$LSAN_BASE %run %t
 // RUN: %env_lsan_opts=$LSAN_BASE not %run %t foo 2>&1 | FileCheck %s
-//
-// UNSUPPORTED: darwin
 
 #include <sanitizer/lsan_interface.h>
 
 int argc_copy;
 
 extern "C" {
-int __lsan_is_turned_off() {
+int __attribute__((used)) __lsan_is_turned_off() {
   return (argc_copy == 1);
 }
 }
 
 int main(int argc, char *argv[]) {
   volatile int *x = new int;
   *x = 42;
   argc_copy = argc;
   return 0;
 }
 
 // CHECK: SUMMARY: {{(Leak|Address)}}Sanitizer: 4 byte(s) leaked in 1 allocation(s)