[Type checker] Eliminate a dynamically-dead diagnostic.

Between syntactic restrictions and later checks for spurious '&'
expressions, we don't need to check for '&' expressions when coercing
to an rvalue. Plus, 'inout' doesn't imply 'not lvalue', so we'd never
get here.

Author: DougGregor

include/swift/AST/DiagnosticsSema.def

@@ -733,8 +733,6 @@ NOTE(previous_precedence_group_decl,none,
 
 ERROR(tuple_conversion_not_expressible,none,
       "cannot express tuple conversion %0 to %1", (Type, Type))
-ERROR(load_of_explicit_lvalue,none,
-      "%0 variable is not being passed by reference", (Type))
 
 //------------------------------------------------------------------------------
 // Expression Type Checking Errors

lib/Sema/TypeCheckConstraints.cpp

@@ -2896,14 +2896,6 @@ Expr *TypeChecker::coerceToRValue(Expr *expr,
     return tryExpr;
   }
 
-  // Can't load from an inout value.
-  if (auto inoutExpr = dyn_cast<InOutExpr>(expr)) {
-    diagnose(expr->getLoc(), diag::load_of_explicit_lvalue,
-             exprTy->castTo<InOutType>()->getObjectType())
-      .fixItRemove(SourceRange(inoutExpr->getLoc()));
-    return coerceToRValue(inoutExpr->getSubExpr(), getType, setType);
-  }
-
   // Walk into tuples to update the subexpressions.
   if (auto tuple = dyn_cast<TupleExpr>(expr)) {
     bool anyChanged = false;