[clang] Verify internal entity module mangling

[urnathan]

Summary

Internal symbol mangling is implementation-defined. We do not mangle any module attachment, and this adds a test to verify that.

Once GCC12 is released, I'll patch GCC to match this (for simplicity, as mentioned, it's not an ABI problem).

[dblaikie]

Sounds alright to me, though I'm not as familiar with the details - no doubt @iains can weigh in if there's anything extra/interesting, even if it's post-commit.

clang/test/CodeGenCXX/cxx20-module-internal.cppm

+// RUN: %clang_cc1 -std=c++20 %s -triple %itanium_abi_triple -emit-llvm -o - | FileCheck %s
+
+// internal-linkage symbol mangling is implementation defined.  Let's
+// not mangle in the module attachment -- that unnecessarily bloats
+// the symbols.
+
+export module A;
+
+// CHECK-DAG: void @_ZL6addonev(
+static void addone() {}
+// CHECK-DAG: @_ZL1x =
+static int x = 5;
+
+namespace {
+// CHECK-DAG: void @_ZN12_GLOBAL__N_14frobEv(
+void frob() {}
+// CHECK-DAG: @_ZN12_GLOBAL__N_11yE =
+int y = 2;
+struct Bill {
+  void F();
+};
+// CHECK-DAG: void @_ZN12_GLOBAL__N_14Bill1FEv(
+void Bill::F() {}
+} // namespace
+
+// CHECK-DAG: void @_ZL4FrobPN12_GLOBAL__N_14BillE(
+static void Frob(Bill *b) {
+  if (b)
+    b->F();
+}
+
+// CHECK-DAG: void @_ZW1A6addsixv(
+void addsix() {
+  Frob(nullptr);
+  frob();
+  addone();
+  void(x + y);
+}