Add a failing test for Clang modules support. (#7233)

Rearrange the file_test infrastructure so that we can customize the
mapping of file names to command line arguments. Map `module.modulemap`
files to corresponding Clang driver flags. In passing, also clean up the
interface for specifying custom argument replacements so that we don't
build a string map for each file we process, and stop using
`SmallVector::insert`.

Assisted-by: Gemini via Antigravity

toolchain/check/testdata/interop/cpp/modules/import.carbon

@@ -0,0 +1,61 @@
+// Part of the Carbon Language project, under the Apache License v2.0 with LLVM
+// Exceptions. See /LICENSE for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+// INCLUDE-FILE: toolchain/testing/testdata/min_prelude/full.carbon
+// EXTRA-ARGS: --clang-arg=-fmodules --clang-arg=-fmodules-cache-path=%t.cache
+//
+// AUTOUPDATE
+// TIP: To test this file alone, run:
+// TIP:   bazel test //toolchain/testing:file_test --test_arg=--file_tests=toolchain/check/testdata/interop/cpp/modules/import.carbon
+// TIP: To dump output, run:
+// TIP:   bazel run //toolchain/testing:file_test -- --dump_output --file_tests=toolchain/check/testdata/interop/cpp/modules/import.carbon
+
+// --- module.modulemap
+
+module A {
+  header "a.h"
+  export *
+}
+
+module B {
+  header "b.h"
+  export *
+}
+
+// --- a.h
+
+struct A {};
+
+A *_Nonnull makeA();
+
+// --- b.h
+
+#include "a.h"
+
+struct B {};
+
+B *_Nonnull makeB(A *_Nonnull);
+
+// --- fail_todo_use_a_b.carbon
+
+library "[[@TEST_NAME]]";
+
+import Cpp library "a.h";
+import Cpp library "b.h";
+
+// CHECK:STDERR: fail_todo_use_a_b.carbon:[[@LINE+4]]:12: error: member name `B` not found in `Cpp` [MemberNameNotFoundInInstScope]
+// CHECK:STDERR: fn Go() -> Cpp.B* {
+// CHECK:STDERR:            ^~~~~
+// CHECK:STDERR:
+fn Go() -> Cpp.B* {
+  // CHECK:STDERR: fail_todo_use_a_b.carbon:[[@LINE+8]]:10: error: member name `makeB` not found in `Cpp` [MemberNameNotFoundInInstScope]
+  // CHECK:STDERR:   return Cpp.makeB(Cpp.makeA());
+  // CHECK:STDERR:          ^~~~~~~~~
+  // CHECK:STDERR:
+  // CHECK:STDERR: fail_todo_use_a_b.carbon:[[@LINE+4]]:20: error: member name `makeA` not found in `Cpp` [MemberNameNotFoundInInstScope]
+  // CHECK:STDERR:   return Cpp.makeB(Cpp.makeA());
+  // CHECK:STDERR:                    ^~~~~~~~~
+  // CHECK:STDERR:
+  return Cpp.makeB(Cpp.makeA());
+}