tsan: clear shadow for ucontext, because it's visible to user
tsan: clear shadow for ucontext, because it's visible to user


git-svn-id: c199f293c43da69278bea8e88f92242bf3aa95f7@159365 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