Fix the AST representation for non-type template arguments to encode

Fix the AST representation for non-type template arguments to encode
enough information so we can mangle them correctly in cases involving
dependent parameter types. (This specifically impacts cases involving
null pointers and cases involving parameters of reference type.)
Fix the mangler to use this information instead of trying to scavenge
it out of the parameter declaration.

<rdar://problem/12296776>.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@164656 91177308-0d34-0410-b5e6-96231b3b80d8