[MSan] Add an isStore argument to getShadowOriginPtr(). NFC

This is a step towards the upcoming KMSAN implementation patch.
The isStore argument is to be used by getShadowOriginPtrKernel(),
it is ignored by getShadowOriginPtrUserspace().

Depending on whether a memory access is a load or a store, KMSAN
instruments it with different functions, __msan_metadata_ptr_for_load_X()
and __msan_metadata_ptr_for_store_X().

Those functions may return different values for a single address,
which is necessary in the case the runtime library decides to ignore
particular accesses.

llvm-svn: 328692