Rename "textual interface" to "parseable interface" (#19713)

We already have something called "module interfaces" -- it's the
generated interface view that you can see in Xcode, the interface
that's meant for developers using a library. Of course, that's also a
textual format. To reduce confusion, rename the new module stability
feature to "parseable [module] interfaces".

test/ModuleInterface/dataflow-errors.swift

@@ -1,12 +0,0 @@
-// 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'
-  }
-}