IRgen improvements on unreachable code:

 - Split out "simple" statements which can easily handle IR generation
   when there is no insert point. These are generally statements which
   start by emitting a new block or are only containers for other
   statements.

 - This fixes a regression in emitting dummy blocks, notably for case
   statements.

 - This also fixes spurious emission of a number of debug stoppoint
   intrinsic instructions.

Remove unneeded sw.body block, just clear the insertion point.

Lift out CodeGenFunction::EmitStopPoint which calls into the
CGDebugInfo class when generating debug info.

Normalize definitions of Emit{Break,Continue}Stmt and usage of
ErrorUnsupported.

llvm-svn: 59118