[compiler-rt] Fix TLS resource leaking in unittest

Summary:
The thread specific key wasn't not released.
Running the unittest in loop will fail after 1024 iteraions.

```
 ./projects/compiler-rt/lib/sanitizer_common/tests/Sanitizer-i386-Test --gtest_filter=SanitizerCommon.PthreadDestructorIterations --gtest_repeat=2000 --gtest_break_on_failure
```

```
Repeating all tests (iteration 1023) . . .

Note: Google Test filter = SanitizerCommon.PthreadDestructorIterations
[==========] Running 1 test from 1 test case.
[----------] Global test environment set-up.
[----------] 1 test from SanitizerCommon
[ RUN      ] SanitizerCommon.PthreadDestructorIterations
/usr/local/google/home/etienneb/llvm/llvm/projects/compiler-rt/lib/sanitizer_common/tests/sanitizer_posix_test.cc:54: Failure
Value of: pthread_key_create(&key, &destructor)
  Actual: 11
Expected: 0
Aborted (core dumped)
```

Reviewers: rnk

Subscribers: kubabrecka, llvm-commits, chrisha

Differential Revision: http://reviews.llvm.org/D21902

llvm-svn: 274264