Specify temporary for this as an input parameter

First commit:

Fix the issue raised in the description of #894: If a method takes an `in` parameter of a struct type that is not a `readonly struct`, then the compiler must create a temporary to invoke a non-readonly member of that type.

Author: BillWagner

standard/expressions.md

@@ -1211,7 +1211,7 @@ The run-time processing of a function member invocation consists of the followin
   - `M` is invoked.
 - Otherwise, if the type of `E` is a value-type `V`, and `M` is declared or overridden in `V`:
   - `E` is evaluated. If this evaluation causes an exception, then no further steps are executed. For an instance constructor, this evaluation consists of allocating storage (typically from an execution stack) for the new object. In this case `E` is classified as a variable.
-  - If `E` is not classified as a variable, then a temporary local variable of `E`’s type is created and the value of `E` is assigned to that variable. `E` is then reclassified as a reference to that temporary local variable. The temporary variable is accessible as `this` within `M`, but not in any other way. Thus, only when `E` is a true variable is it possible for the caller to observe the changes that `M` makes to `this`.
+  - If `E` is not classified as a variable, or if `E` is an input parameter (§15.6.2.3) and `V` is not a readonly struct type (§16.2.2), then a temporary local variable of `E`’s type is created and the value of `E` is assigned to that variable. `E` is then reclassified as a reference to that temporary local variable. The temporary variable is accessible as `this` within `M`, but not in any other way. Thus, only when `E` is a true variable is it possible for the caller to observe the changes that `M` makes to `this`.
   - The argument list is evaluated as described in [§12.6.2](expressions.md#1262-argument-lists).
   - `M` is invoked. The variable referenced by `E` becomes the variable referenced by `this`.
 - Otherwise: