[Sema] Add a comment on an identified bug on default arguments.

Summary:
The mis-compile is triggered by internal code, but I haven't reduced it to a small piece of code. Add a FIXME here, since a decent fix doesn't seem to be trivial.

The decent fix can be changing Decl::Init to PointerUnion<Stmt *, EvaluatedStmt *, ParamVarDecl *>, and make setUninstantiatedDefaultArg take a ParamVarDecl *, which contains the Expr * as the default argument. This way, getTemplateInstantiationArgs can take that ParamVarDecl and do the right thing.

Reviewers: rsmith

Subscribers: sanjoy, cfe-commits

Differential Revision: https://reviews.llvm.org/D36253

llvm-svn: 309908