[MSan] fix the types of RegSaveAreaPtrPtr and OverflowArgAreaPtrPtr

Despite their names, RegSaveAreaPtrPtr and OverflowArgAreaPtrPtr
used to be i8* instead of i8**.

This is important, because these pointers are dereferenced twice
(first in CreateLoad(), then in getShadowOriginPtr()), but for some
reason MSan allowed this - most certainly because it was possible
to optimize getShadowOriginPtr() away at compile time.

Differential revision: https://reviews.llvm.org/D44520

llvm-svn: 327830