Doxycomment SVal's getAs/castAs.

Also document TypeLoc's operations similarly, since it's a good idea.

Post-commit CR feedback from Anna Zaks regarding r175594.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@175694 91177308-0d34-0410-b5e6-96231b3b80d8
Mark unimplemented function with LLVM_DELETED_FUNCTION

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@175695 91177308-0d34-0410-b5e6-96231b3b80d8