Add name_reference SemIR node for references to names. (#3260)

Also add `name_reference_untyped` for references to non-first-class
names without types, which currently covers namespaces and functions.

This improves the fidelity of the SemIR representation, and fixes some
issues where we would use the wrong location for nodes and diagnostics
downstream of a name reference.

We're still missing a representation for dotted name expressions, such
as `Namespace.Function`, and we don't use the `untyped` node as an
operand of any other node yet.

toolchain/check/testdata/function/declaration/simple.carbon

@@ -17,6 +17,7 @@ fn G() { F(); }
 // CHECK:STDOUT:
 // CHECK:STDOUT: fn @G() {
 // CHECK:STDOUT: !entry:
+// CHECK:STDOUT:   %F.ref = name_reference_untyped "F", package.%F
 // CHECK:STDOUT:   %.loc9_11.1: type = tuple_type ()
 // CHECK:STDOUT:   %.loc9_11.2: init () = call @F()
 // CHECK:STDOUT:   return