Explorer: rename value categories to expression categories (#2744)

Rename value categories to expression categories based on [Discord discussion](https://discord.com/channels/655572317891461132/753021843459538996/1092924035517665332) regarding naming and behavior.

>* let expression -> value expression
>* var expression -> reference expression
>* located expression -> initializing expression
>So:
>- "value expressions" produce values (with no associated location). "reference expressions" produce a location of an existing value. "initializing expressions" take a location and initialize it.
>- A let binding is initialized by a value expression, because lets represent values (with category conversions performed as needed, but if a conversion is performed from a different category of expression, the value of the object is pinned for the lifetime of the let).
>- A var binding is initialized by an initializing expression, without performing a copy (with category conversions performed as needed, calling a copy constructor if the initializer is a different expression category).
>- The & operator requires a reference expression, and it's an error to give it other kinds.
>- The left-hand side of . requires a value expression when calling a function with a non-addr receiver, and requires a reference expression when calling a function with an addr receiver (it's an error to give it a value expression, and for an initializing expression, a temporary is materialized).

Changes
* Rename "value category" to "expression category"
* Rename Var and Let value categories to Value, Reference, and Initializing expression
* Rename `lvalue` to `location` (most of the time)

explorer/testdata/basic_syntax/fail_assign_to_function.carbon

@@ -12,7 +12,7 @@ fn F() {}
 fn G() {}
 
 fn Main() -> i32 {
-  // CHECK:STDERR: COMPILATION ERROR: {{.*}}/explorer/testdata/basic_syntax/fail_assign_to_function.carbon:[[@LINE+1]]: Cannot assign to rvalue 'F'
+  // CHECK:STDERR: COMPILATION ERROR: {{.*}}/explorer/testdata/basic_syntax/fail_assign_to_function.carbon:[[@LINE+1]]: Only assign a reference expression can be assigned, but got `'F`'
   F = G;
   return 0;
 }