test: a warning on any compile path must not fail the build

The fixture puts a #warning in each kind of source the build compiles —
module interface, module implementation, C, consumer — plus an unknown -z
value so ld.lld warns on the link step too, and asserts the build succeeds
cold (every unit compiling, every warning emitted), succeeds with nothing to
do, and succeeds again after each source is touched back into staleness.
The staged copy lives outside the repo because the whole test depends on
units compiling for the first time.

The last pass breaks a source on purpose: "never fails" would satisfy
everything above just as well, so the error path is asserted too — both that
it fails and that it still reports the compiler's diagnostic.

tests/CompilerWarning/fixture/main.cpp

@@ -0,0 +1,17 @@
+// SPDX-License-Identifier: LGPL-3.0-only
+// SPDX-FileCopyrightText: Copyright (C) 2026 Catcrafts®
+
+#warning "consumer is noisy"
+
+import std;
+import Widget;
+
+extern "C" long CounterValue();
+
+// Prints "<module value> <C value>". The test asserts on it so that "the build
+// succeeded" also means the objects behind those two numbers were really
+// produced, not merely left over from an earlier pass.
+int main() {
+    std::print("{} {}", WidgetValue(), CounterValue());
+    return 0;
+}