[test] Avoid relying on stdlib in serialized-diagnostics-prettyprint.swift

Rather than relying on the presence of particular String initializers,
emit a module with a type with an overloaded initializer.

Author: hamishknight

test/Misc/serialized-diagnostics-prettyprint.swift

@@ -1,14 +1,32 @@
-// RUN: rm -f %t.*
+// RUN: %empty-directory(%t)
+// RUN: split-file %s %t
 
-// Test swift executable
-// RUN: %target-swift-frontend -typecheck -serialize-diagnostics-path %t.dia %s -verify
-// RUN: c-index-test -read-diagnostics %t.dia > %t.deserialized_diagnostics.txt 2>&1
-// RUN: %FileCheck --input-file=%t.deserialized_diagnostics.txt %s
+// RUN: %target-swift-frontend -emit-module %t/Lib.swift -module-name Lib -emit-module-path %t/Lib.swiftmodule
+// RUN: %target-swift-frontend -typecheck -I %t -serialize-diagnostics-path %t/diags.dia %t/Client.swift -verify
+// RUN: c-index-test -read-diagnostics %t/diags.dia > %t/diags.deserialized_diagnostics.txt 2>&1
+// RUN: %FileCheck --input-file=%t/diags.deserialized_diagnostics.txt %t/Client.swift
 
-var x = String.init(_:) // expected-error{{ambiguous use of 'init(_:)'}}
-// CHECK: {{.*[/\\]}}serialized-diagnostics-prettyprint.swift:[[@LINE-1]]:16: error: ambiguous use of 'init(_:)'
+//--- Lib.swift
 
-// CHECK: Swift.String.init:2:19: note: found this candidate
-// CHECK: CONTENTS OF FILE Swift.String.init:
-// CHECK: struct String {
-// CHECK:    public init(_ content: Substring.UnicodeScalarView)
+public struct S {
+  public init(a: String) {}
+  @discardableResult // We use @discardableResult here to give this init a distinct line number in the pretty print.
+  public init(b: Int) {}
+}
+
+//--- Client.swift
+
+import Lib
+
+var x = S.init // expected-error {{ambiguous use of 'init'}}
+// CHECK: {{.*[/\\]}}Client.swift:[[@LINE-1]]:11: error: ambiguous use of 'init'
+
+// FIXME: Currently we can't check for 'init(a:)' because c-index-test is
+// keying the source file contents on the buffer identifier, so we end up
+// with duplicated 'init(b:)'.
+
+// CHECK: Lib.S.init:3:10: note: found this candidate
+// CHECK:      CONTENTS OF FILE Lib.S.init:
+// CHECK:      struct S {
+// CHECK-NEXT:    @discardableResult
+// CHECK-NEXT:    public init(b: Int)