Remove RuntimeError / CompilationError. (#2258)

Instead, work out the prefix for an error based on whether it was produced during parsing, semantic analysis, or when running the program.

explorer/ast/expression.cpp

@@ -45,7 +45,7 @@ auto IntrinsicExpression::FindIntrinsic(std::string_view name,
   name.remove_prefix(std::strlen("__intrinsic_"));
   auto it = intrinsic_map.find(name);
   if (it == intrinsic_map.end()) {
-    return CompilationError(source_loc) << "Unknown intrinsic '" << name << "'";
+    return ProgramError(source_loc) << "Unknown intrinsic '" << name << "'";
   }
   return it->second;
 }