Allow the warning 'case value not in enumerated type' to be silenced with
the following pattern.

If 'case' expression refers to a static const variable of the correct enum
type, then we count this as a sufficient declaration of intent by the user,
so we silence the warning.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@196546 91177308-0d34-0410-b5e6-96231b3b80d8