[lsan] Fix flaky test in swapcontext.cpp (#175635)

LSan, by design, can have false negatives, making it unreliable to check
that the leak was found in the stack-allocated case:
```
==123685==Scanning STACK range 0x7ffe6e554ca0-0x7ffe6e557000.
==123685==0x7ffe6e554de0: found 0x51e0000009f0 pointing into chunk 0x51e000000000-0x51e000000c00 of size 3072.
==123685==0x7ffe6e554e30: found 0x51e000000c00 pointing into chunk 0x51e000000c00-0x51e000001668 of size 2664. <- this prevented the leak from being found
```

This has led to flakiness on the buildbots e.g.,
https://lab.llvm.org/buildbot/#/builders/66/builds/24669
```
# | /home/b/sanitizer-x86_64-linux/build/llvm-project/compiler-rt/test/lsan/TestCases/swapcontext.cpp:44:11: error: CHECK: expected string not found in input
# | // CHECK: SUMMARY: {{.*}}Sanitizer: 2664 byte(s) leaked in 1 allocation(s)
...
Failed Tests (2):
  LeakSanitizer-HWAddressSanitizer-x86_64 :: TestCases/swapcontext.cpp
  LeakSanitizer-Standalone-x86_64 :: TestCases/swapcontext.cpp
```

This patch fixes the issue by clearing the buffer, as suggested by
Vitaly.