tsan: add a hook to obtain number of reports
tsan: add a hook to obtain number of reports

Requested in:
https://github.com/golang/go/issues/15972
Will help to fail individual test cases with races.



git-svn-id: c199f293c43da69278bea8e88f92242bf3aa95f7@285455 91177308-0d34-0410-b5e6-96231b3b80d8

llvm-mirror/compiler-rt,llvm-mirror/compiler-rt,llvm-mirror/compiler-rt,llvm-mirror/compiler-rt,llvm-mirror/compiler-rt