[LSan] Rewrite the test from r223419 to not use C++11.

This was causing build failures on llvm-clang-lld-x86_64-centos-6.5 for some
reason. Anyway, the new way is better because we no longer rely on std::thread
implementation details.

git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@223480 91177308-0d34-0410-b5e6-96231b3b80d8
[ASan] Refactor thread starting code.

Move thread context creation into AsanThread::Create().

git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@223483 91177308-0d34-0410-b5e6-96231b3b80d8