Add "KnownSVal" to represent SVals that cannot be UnknownSVal.

This provides a few sundry cleanups, and allows us to provide
a compile-time check for a case that was a runtime assertion.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@175987 91177308-0d34-0410-b5e6-96231b3b80d8
4 files changed