Provide diagnostic locations for imported namespaces. (#3640)

Building on #3636 which handles the general import case, add special
casing for namespaces. Namespaces can be combined cross-IR, so it's a
little more complex.

The implementation adds import_id to the Namespace instruction as a
reference to find the original using the normal structure. This is
achieved by moving the name_id to NameScope to free up space.

---

I considered a few alternatives...

I considered adding import_id to NameScope, but:

1. It's more consistent with things such as Function or Class that
provide name_id on the info object rather than the instruction.
2. I thought it more likely that there would be more NameScope cases
that might want a name_id rather than the import_id, since ImportRef
will typically be used.

I considered putting the import source (cross-ref IR id + inst id) on
the NameScope versus a separate ImportRef, which seems like the
strongest argument towards the NameScope approach because it removes an
instruction. That just felt inconsistent though, and the overhead of
instruction-per-imported-namespace should be low (theoretically few
namespaces should be used). Plus I feel a bit odd adding two
generally-unused ids to NameScope.

A specialized Namespace structure could also have been created to store
the import_id, but that would add an indirection to the NameScope.

toolchain/check/testdata/array/array_vs_tuple.carbon

@@ -29,7 +29,7 @@ fn G() {
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
 // CHECK:STDOUT: file {
-// CHECK:STDOUT:   package: <namespace> = namespace package, {.G = %G} [template]
+// CHECK:STDOUT:   package: <namespace> = namespace {.G = %G} [template]
 // CHECK:STDOUT:   %G: <function> = fn_decl @G [template]
 // CHECK:STDOUT: }
 // CHECK:STDOUT: