Add two new memory region based checks to the Unwinder:

Check that the pc value for frames up the stack is in a
mapped+executable region of memory.

Check that the stack pointer for frames up the stack is
in a mapped+readable region of memory.

If the unwinder ever makes a mistake walking the stack,
these checks will help to keep it from going too far into
the weeds.

These aren't fixing any bugs that I know of, but they
add extra robustness to a complicated task.

llvm-svn: 146478