Initial support for return forms (#6556)

The main changes here are:
- Introducing `InitForm` and `RefForm` to represent initializing and
reference forms (the two return forms currently supported by the
parser).
- Introducing the `FormType` singleton inst to represent their type
(i.e. `Core.Form`).
- Emitting an inst representing a function's declared return form as
part of handling the function signature.

The return form inst is currently ignored. Subsequent PRs will expose it
in `SemIR::Function` and use it to determine the form of call
expressions.

toolchain/check/testdata/pointer/basic.carbon

@@ -28,6 +28,7 @@ fn F() -> i32 {
 // CHECK:STDOUT:   %Int.generic: %Int.type = struct_value () [concrete]
 // CHECK:STDOUT:   %N: Core.IntLiteral = symbolic_binding N, 0 [symbolic]
 // CHECK:STDOUT:   %i32: type = class_type @Int, @Int(%int_32) [concrete]
+// CHECK:STDOUT:   %.941: form = init_form %i32, call_param0 [concrete]
 // CHECK:STDOUT:   %pattern_type.7ce: type = pattern_type %i32 [concrete]
 // CHECK:STDOUT:   %F.type: type = fn_type @F [concrete]
 // CHECK:STDOUT:   %F: %F.type = struct_value () [concrete]
@@ -109,6 +110,7 @@ fn F() -> i32 {
 // CHECK:STDOUT:   } {
 // CHECK:STDOUT:     %int_32.loc15: Core.IntLiteral = int_value 32 [concrete = constants.%int_32]
 // CHECK:STDOUT:     %i32.loc15: type = class_type @Int, @Int(constants.%int_32) [concrete = constants.%i32]
+// CHECK:STDOUT:     %.loc15: form = init_form %i32.loc15, call_param0 [concrete = constants.%.941]
 // CHECK:STDOUT:     %return.param: ref %i32 = out_param call_param0
 // CHECK:STDOUT:     %return: ref %i32 = return_slot %return.param
 // CHECK:STDOUT:   }