[analyzer] pr38668: Do not attempt to cast loaded values of non-scalar types.

It is expected to have the same object (memory region) treated as if it has
different types in different program points. The correct behavior for
RegionStore when an object is stored as an object of type T1 but loaded as
an object of type T2 is to store the object as if it has type T1 but cast it
to T2 during load.

Note that the cast here is some sort of a "reinterpret_cast" (even in C). For
instance, if you store a float and load an integer, you won't have your float
rounded to an integer; instead, you will have garbage.

Admit that we cannot perform the cast as long as types we're dealing with are
non-trivial (neither integers, nor pointers).

Of course, if the cast is not necessary (eg, T1 == T2), we can still load the
value just fine.

Differential Revision: https://reviews.llvm.org/D55875

rdar://problem/45062567

llvm-svn: 349701