Add regression test for private cross-module constructor (#7152)

Calling a private single-parameter constructor of a struct defined in a
separate module used to segfault. It now reports an accessibility error;
add a regression test to lock that in.

Fixes #7152.

Author: expipiplus1

tests/diagnostics/private-init-cross-module-helper.slang

@@ -0,0 +1,9 @@
+module "private-init-cross-module-helper";
+
+public struct B
+{
+    // A private single-parameter constructor in a separate module. Calling
+    // this from another module used to crash the compiler (see issue #7152);
+    // it must now produce an accessibility diagnostic instead.
+    __init(float b) {}
+}

tests/diagnostics/private-init-cross-module.slang

@@ -0,0 +1,18 @@
+// Regression test for issue #7152: calling a private single-parameter
+// constructor of a struct defined in a separate module used to segfault.
+// It must now report an accessibility error rather than crash.
+
+//DIAGNOSTIC_TEST:SIMPLE(diag=CHECK):-target spirv
+
+import "private-init-cross-module-helper.slang";
+
+[shader("vertex")]
+float4 foo() : SV_Position
+{
+    B bar = B(0.0);
+//CHECK:     ^ declaration not accessible
+//CHECK:     ^ 'B.init' is not accessible from the current context.
+//CHECK:      ^ type mismatch in expression
+//CHECK:      ^ expected an expression of type 'B', got 'float'
+    return float4(1, 1, 1, 1);
+}