C++ PATCH for c++/47904 (crash with template argument that uses 'this')

[Jason Merrill]

When I added DECL_PARM_INDEX, it didn't occur to me that we could hit 
the code for hashing/comparing args with 'this' when used inside a 
function; I was only thinking of uses in a trailing return type, where 
'this' isn't in scope (well, wasn't...that is changing).

And while I was fixing that I also fixed the code to handle the new 
DECL_PARM_LEVEL as it should.

Tested x86_64-pc-linux-gnu, applied to trunk.  I'll backport to 4.5 soon.
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