Document the value casting API changes.

After changing the way several value inheritance hierarchies (TypeLoc,
CFGelement, ProgramPoint and SVal) handle casting, this documentation describes
how 3rd party code may need to be updated to compile with the new APIs.

As suggested by Sean Silva on cfe-dev.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@175970 91177308-0d34-0410-b5e6-96231b3b80d8