SafeStack: Add builtins to read unsafe stack top/bottom

Summary:
Introduce built-ins to read the unsafe stack top and bottom. The unsafe
stack top is required to implement garbage collection scanning for
Oilpan. Currently there is already a built-in 'get_unsafe_stack_start'
to read the bottom of the unsafe stack, but I chose to duplicate this
API because 'start' is ambiguous (e.g. Oilpan uses WTF::GetStackStart to
read the safe stack top.)

Reviewers: pcc

Reviewed By: pcc

Subscribers: llvm-commits, kcc

Differential Revision: https://reviews.llvm.org/D49152

llvm-svn: 337037