[analyzer] Track malloc'd memory into struct fields.

Due to improper modelling of copy constructors (specifically, their
const reference arguments), we were producing spurious leak warnings
for allocated memory stored in structs. In order to silence this, we
decided to consider storing into a struct to be the same as escaping.
However, the previous commit has fixed this issue and we can now properly
distinguish leaked memory that happens to be in a struct from a buffer
that escapes within a struct wrapper.

Originally applied in r161511, reverted in r174468.
<rdar://problem/12945937>

llvm-svn: 177571