[analyzer] Support partially tainted records.

The analyzer's taint analysis can now reason about structures or arrays
originating from taint sources in which only certain sections are tainted.

In particular, it also benefits modeling functions like read(), which may
read tainted data into a section of a structure, but RegionStore is incapable of
expressing the fact that the rest of the structure remains intact, even if we
try to model read() directly.

Patch by Vlad Tsyrklevich!

Differential revision: https://reviews.llvm.org/D28445

llvm-svn: 304162