[clang-doc] Add test case for #141990 (#142209)

When we landed the fix for the assertion in #141990, we hadn't yet
reduced the test case sufficiently for a regression test.

Author: ilovepi

clang-tools-extra/test/clang-doc/DR-141990.cpp

@@ -0,0 +1,18 @@
+// RUN: rm -rf %t && mkdir -p %t
+// RUN: clang-doc -output=%t %s 2>&1 | FileCheck %s --implicit-check-not="{{warning|error}}"
+
+// COM: This case triggered an assertion before #141990:
+// COM: clang-doc: llvm-project/clang/lib/AST/Decl.cpp:2985:
+// COM:   Expr *clang::ParmVarDecl::getDefaultArg(): Assertion `!hasUninstantiatedDefaultArg()
+// COM:   && "Default argument is not yet instantiated!"' failed.
+
+template <class = int>
+class c;
+int e;
+
+template <class>
+class c {
+public:
+  void f(int n = e);
+};
+class B : c<> {};