[Frontend] Allow -emit-interface with -typecheck (#19676)

Textual module interfaces don't actually depend on SILGen, so we
shouldn't need to run SILGen (or serialize an entire binary module) if
we're just trying to emit a textual interface. On the other hand, if
we /are/ going to run SILGen and then SIL diagnostics, we shouldn't
delay those diagnostics by spending time emitting a textual interface,
or for that matter a TBD file.

Using this, update all the ModuleInterface tests that use
`-emit-module -o /dev/null` to use `-typecheck` instead, except for
those using `-merge-modules`.

test/ModuleInterface/dataflow-errors.swift

@@ -0,0 +1,12 @@
+// RUN: rm -f %t
+// RUN: not %target-swift-frontend -emit-interface-path %t -emit-module -o /dev/null %s
+// RUN: test ! -f %t
+// RUN: %target-swift-frontend -emit-interface-path %t -typecheck %s
+// RUN: test -f %t
+
+public struct BadInit {
+  public var x: Int
+  public init() {
+    return // without initializing 'x'
+  }
+}