[Sanitizer] Add generic ThreadRegistry class for sanitizer runtimes. This class holds basic thread bookkeeping logic and allows specific sanitizer runtimes to create thread contexts and mark threads as created/running/joined etc. The class is based on the way we currently store thread contexts in TSan.

git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@177074 91177308-0d34-0410-b5e6-96231b3b80d8