Prefer 'and' over '&' in comments.

Post commit code review feedback from Matt Beaumont-Gay on r174248.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@175969 91177308-0d34-0410-b5e6-96231b3b80d8
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
Reference the canonical commit (r175462) in the value casting release note.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@175971 91177308-0d34-0410-b5e6-96231b3b80d8