[C++20] [Modules] Add signature to the BMI recording export imported

modules

After https://github.com/llvm/llvm-project/pull/86912,
for the following example,

```
export module A;
export import B;
```

The generated BMI of `A` won't change if the source location in `A`
changes. Further, we plan avoid more such changes.

However, it is slightly problematic since `export import` should
propagate all the changes.

So this patch adds a signature to the BMI of C++20 modules so that we
can propagate the changes correctly.

1 files changed, 38 insertions, 0 deletions

diff --git a/clang/test/Modules/force-transitive-changes.cppm b/clang/test/Modules/force-transitive-changes.cppm
new file mode 100644
index 000000000000..5732d264d1d5
--- /dev/null
+++ b/clang/test/Modules/force-transitive-changes.cppm
@@ -0,0 +1,38 @@
+// Test that the changes from export imported modules and touched
+// modules can be popullated as expected.
+//
+// RUN: rm -rf %t
+// RUN: split-file %s %t
+// RUN: cd %t
+//
+// RUN: %clang_cc1 -std=c++20 %t/A.cppm -emit-reduced-module-interface -o %t/A.pcm
+// RUN: %clang_cc1 -std=c++20 %t/A.v1.cppm -emit-reduced-module-interface -o %t/A.v1.pcm
+
+// The BMI of B should change it export imports A, so all the change to A should be popullated
+// to B.
+// RUN: %clang_cc1 -std=c++20 %t/B.cppm -emit-reduced-module-interface -fmodule-file=A=%t/A.pcm \
+// RUN:     -o %t/B.pcm
+// RUN: %clang_cc1 -std=c++20 %t/B.cppm -emit-reduced-module-interface -fmodule-file=A=%t/A.v1.pcm \
+// RUN:     -o %t/B.v1.pcm
+// RUN: not diff %t/B.v1.pcm %t/B.pcm  &> /dev/null
+
+//--- A.cppm
+export module A;
+export int funcA() {
+    return 43;
+}
+
+//--- A.v1.cppm
+export module A;
+
+export int funcA() {
+    return 43;
+}
+
+//--- B.cppm
+export module B;
+export import A;
+
+export int funcB() {
+    return funcA();
+}