Fix repainting of compile-time reals for targets with 64-bit reals

By converting the real_t to a `double` and repainting that value.
E.g., this fixes druntime-test-hash for MSVC targets;
core.internal.hash.hashOf!(real) reinterprets a 64-bit (target) `real`
as `ulong`, and that code path is taken at CTFE too.

dmd/compiler.d

@@ -160,6 +160,9 @@ else
         case Tfloat64:
             u.float64value = cast(double) e.toReal();
             break;
+        case Tfloat80:
+            assert(e.type.size() == 8); // 64-bit target `real`
+            goto case Tfloat64;
         default:
             assert(0, "Unsupported source type");
         }
@@ -187,6 +190,10 @@ else
             emplaceExp!(RealExp)(pue, e.loc, r, type);
             break;
 
+        case Tfloat80:
+            assert(type.size() == 8); // 64-bit target `real`
+            goto case Tfloat64;
+
         default:
             assert(0, "Unsupported target type");
         }