[Frontend] Add test that we can create a module if same filename is used twice and -experimental-allow-module-with-compiler-errors is passed

Author: ahoppen

test/Frontend/Inputs/same_filename/A/File.swift

@@ -0,0 +1 @@
+class Foo {}

test/Frontend/Inputs/same_filename/B/File.swift

@@ -0,0 +1 @@
+class Bar {}

test/Frontend/same_filename_twice.swift

@@ -0,0 +1,13 @@
+// RUN: %empty-directory(%t)
+
+// A module should fail to be generated if the same filename is used twice and '-experimental-allow-module-with-compiler-errors' is not passed
+
+// RUN: not %target-swift-frontend -emit-module -o %t/no_allow_compiler_errors.swiftmodule %S/Inputs/same_filename/A/File.swift %S/Inputs/same_filename/B/File.swift
+// RUN: not ls %t/no_allow_compiler_errors.swiftmodule
+
+// If '-experimental-allow-module-with-compiler-errors' is passed, we should throw an error but still generate a module
+
+// RUN: %target-swift-frontend -emit-module -experimental-allow-module-with-compiler-errors -o %t/allow_compiler_errors.swiftmodule %S/Inputs/same_filename/A/File.swift %S/Inputs/same_filename/B/File.swift 2>&1 | %FileCheck %s
+// RUN: ls %t/allow_compiler_errors.swiftmodule
+
+// CHECK: filename "File.swift" used twice: