use singleton expr to generate black hole errors

this also reduces forceValue code size and removes the need for
hideInDiagnostics. coopting thunk forcing like this has the additional
benefit of clarifying how these errors can happen in the first place.

src/libexpr/nixexpr.cc

@@ -9,6 +9,8 @@
 
 namespace nix {
 
+ExprBlackHole eBlackHole;
+
 struct PosAdapter : AbstractPos
 {
     Pos::Origin origin;