With modules, serialization might omit the outer ExprWithCleanups
as it calls ParmVarDecl::getDefaultArg(). Complementary to fixing
this in a separate change, make the code more robust by adding a
FullExpressionRAII and avoid the llvm_unreachable in the added test
clang/test/Modules/pr68702.cpp.
Closes https://github.com/llvm/llvm-project/issues/68702
844f8335f2
