Disable thread safety analysis for some functions in __thread_support

Many thread-related libc++ test cases fail on FreeBSD, due to the
following -Werror warnings:

    In file included from test/std/thread/thread.threads/thread.thread.this/sleep_until.pass.cpp:17:
    In file included from include/thread:97:
    In file included from include/__mutex_base:17:
    include/__threading_support:222:1: error: mutex '__m' is still held at the end of function [-Werror,-Wthread-safety-analysis]
    }
    ^
    include/__threading_support:221:10: note: mutex acquired here
      return pthread_mutex_lock(__m);
             ^
    include/__threading_support:231:10: error: releasing mutex '__m' that was not held [-Werror,-Wthread-safety-analysis]
      return pthread_mutex_unlock(__m);
             ^
    include/__threading_support:242:1: error: mutex '__m' is still held at the end of function [-Werror,-Wthread-safety-analysis]
    }
    ^
    include/__threading_support:241:10: note: mutex acquired here
      return pthread_mutex_lock(__m);
             ^
    include/__threading_support:251:10: error: releasing mutex '__m' that was not held [-Werror,-Wthread-safety-analysis]
      return pthread_mutex_unlock(__m);
             ^
    include/__threading_support:272:10: error: calling function 'pthread_cond_wait' requires holding mutex '__m' exclusively [-Werror,-Wthread-safety-analysis]
      return pthread_cond_wait(__cv, __m);
             ^
    include/__threading_support:278:10: error: calling function 'pthread_cond_timedwait' requires holding mutex '__m' exclusively [-Werror,-Wthread-safety-analysis]
      return pthread_cond_timedwait(__cv, __m, __ts);
             ^
    6 errors generated.

This is because on FreeBSD, the pthread functions have lock annotations.
Since the functions in __thread_support are internal to libc++ only, add
no_thread_safety_analysis attributes to suppress these warnings.

Reviewers: mclow.lists, EricWF, delesley, aaron.ballman
Reviewed By: aaron.ballman
Subscribers: ed, aaron.ballman, joerg, emaste, cfe-commits
Differential Revision: https://reviews.llvm.org/D28520

llvm-svn: 293197