[memprof] Update the frame is inline logic and unittests.

Since DI frames are enumerated with the leaf function at index 0, this
patch fixes the logic when IsInlineFrame is set. Also update the
unittests to check that only the last frame is marked as non-inline from
a set of DI Frames for a PC address.

Differential Revision: https://reviews.llvm.org/D121830