Refactor scope building in JumpDiagnostics for simplicity. This fixes a

(currently theoretical) bug where recursive calls to BuildScopeInformation
would do the wrong thing if the type of the statement is one of the kinds with
special handling, but this is not currently observable because the relevant
recursive calls happen to all be for CompoundStmts. (This becomes visible with
the C++17 'constexpr if' feature, where we get a protected scope for the 'then'
/ 'else' cases of some 'if's, and don't necessarily have a corresponding
compound statement.)

llvm-svn: 273309