[ModuleInterface] Don't include 'import Builtin' for the stdlib (#19319)

The special 'Builtin' module is implicitly imported via -parse-stdlib,
and can't be found via normal import resolution, so we shouldn't print
it in a textual interface.

test/ModuleInterface/stdlib.swift

@@ -0,0 +1,10 @@
+// RUN: %target-swift-frontend -emit-interface-path - -emit-module -o /dev/null -parse-stdlib %s | %FileCheck %s
+
+// CHECK-NOT: import Builtin
+
+// CHECK: func test() {
+// CHECK-NEXT: Builtin.sizeof
+// CHECK-NEXT: {{^}$}}
+@inlinable public func test() {
+  Builtin.sizeof(Builtin.Int8.self)
+}