Promote FunctionType to a standard instruction. (#3931)

This removes the builtin FunctionType, replacing it with a FunctionType
instruction. The constant for a FunctionDecl is now a StructValue with
type of FunctionType.

Note this means a function declaration produces _both_ a type, and a
value of the type. This has some consequences in terms of circularity,
and makes the importing of function declarations a little more complex.

It'll get particularly peculiar for imports because of the behavior of
the reference, but that's a known issue due to other things such as
`alias`. The impact will hopefully be contained to
ResolvePrevInstForMerge (and ImportRefs).

To note a small formatting change in diagnostics:

```
-  // CHECK:STDERR: fail_member_lookup.carbon:[[@LINE+4]]:3: ERROR: Value of type `<associated <function> in Interface>` is not callable.
+  // CHECK:STDERR: fail_member_lookup.carbon:[[@LINE+4]]:3: ERROR: Value of type `<associated F in Interface>` is not callable.

-  // CHECK:STDERR: fail_todo_facet_lookup.carbon:[[@LINE+4]]:3: ERROR: Value of type `<associated <function> in Interface>` is not callable.
+  // CHECK:STDERR: fail_todo_facet_lookup.carbon:[[@LINE+4]]:3: ERROR: Value of type `<associated F in Interface>` is not callable.
```

---------

Co-authored-by: Richard Smith <richard@metafoo.co.uk>

toolchain/lower/testdata/impl/instance_method.carbon

@@ -32,6 +32,6 @@ fn Call(a: A*) -> A* {
 // CHECK:STDOUT:
 // CHECK:STDOUT: define ptr @Call(ptr %a) {
 // CHECK:STDOUT: entry:
-// CHECK:STDOUT:   %.loc22_18 = call ptr @Get(ptr %a)
-// CHECK:STDOUT:   ret ptr %.loc22_18
+// CHECK:STDOUT:   %Get.call = call ptr @Get(ptr %a)
+// CHECK:STDOUT:   ret ptr %Get.call
 // CHECK:STDOUT: }