C++ inteop: Set type source info for a generated C++ thunk function (#6049)

This prevents a null pointer access crash when generating a thunk with
an automatically deduced trivial return type.
In this case, Clang calls `Sema::DeduceFunctionTypeFromReturnExpr()`
which calls `Sema::getReturnTypeLoc()`, which requires this information.

Part of #5514.

toolchain/check/cpp_thunk.cpp

@@ -335,6 +335,10 @@ static auto CreateThunkFunctionDecl(
                                *callee_info.decl),
       clang_loc));
 
+  // Set function declaration type source info.
+  thunk_function_decl->setTypeSourceInfo(ast_context.getTrivialTypeSourceInfo(
+      thunk_function_decl->getType(), clang_loc));
+
   return thunk_function_decl;
 }