Rename TypeLoc's isType to isKind

Matches changes made to SVal's similar functions based on Jordan Rose's review
feedback to r175594.

Also change isKind to take a reference rather than a non-null pointer, while I'm
at it. (& make TypeLoc::isKind private)

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@175704 91177308-0d34-0410-b5e6-96231b3b80d8
Use None rather than Optional<T>() where possible.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@175705 91177308-0d34-0410-b5e6-96231b3b80d8