asan/tsan: weak interceptors

The idea isthat asan/tsan can survive if user intercepts the same functions. At the same time user has an ability to call back into asan/tsan runtime. See the following tests for examples:
asan/output_tests/interception_failure_test-linux.cc
asan/output_tests/interception_test-linux.cc
asan/output_tests/interception_malloc_test-linux.cc

llvm-svn: 157388