[CaptureTracking] Handle capturing of launder.invariant.group

Summary:
launder.invariant.group has the same rules of capturing as
bitcast, gep, etc - the original value is not captured
if the returned pointer is not captured.

With this patch, we mark 40% more functions as noalias when compiling with -fstrict-vtable-pointers;
1078 vs 1778  (39.37%)

Reviewers: sanjoy, davide, nlewycky, majnemer, mehdi_amini

Subscribers: JDevlieghere, llvm-commits

Differential Revision: https://reviews.llvm.org/D32673

llvm-svn: 331587