For llvm::sys::ThreadLocalImpl instead of malloc'ing the platform-specific

For llvm::sys::ThreadLocalImpl instead of malloc'ing the platform-specific
thread local data, embed them in the class using a uint64_t and make sure
we get compiler errors if there's a platform where this is not big enough.

This makes ThreadLocal more safe for using it in conjunction with CrashRecoveryContext.

Related to crash in rdar://11434201.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@158342 91177308-0d34-0410-b5e6-96231b3b80d8