[clang-doc] Fix assertions error in Serialize.cpp

We can only print and use a default arg, if it is instantiated.
I was unable to reduce the test case for this to something of reasonable
size, but this is easily hit by running clang-doc to generate clang's
documentation. For now, we can fix the assertion quickly to unbreak
users, and add the proper test once it is small enough.

Author: ilovepi

clang-tools-extra/clang-doc/Serialize.cpp

@@ -113,9 +113,8 @@ getFunctionPrototype(const FunctionDecl *FuncDecl) {
       Stream << " " << ParamDecl->getNameAsString();
 
     // Print default argument if it exists
-    if (ParamDecl->hasDefaultArg()) {
-      const Expr *DefaultArg = ParamDecl->getDefaultArg();
-      if (DefaultArg) {
+    if (ParamDecl->hasDefaultArg() && !ParamDecl->hasUninstantiatedDefaultArg()) {
+      if(const Expr *DefaultArg = ParamDecl->getDefaultArg()) {
         Stream << " = ";
         DefaultArg->printPretty(Stream, nullptr, Ctx.getPrintingPolicy());
       }