C++ PATCH for c++/54653 (ICE with missing template-header)

[Jason Merrill]

In this PR, we have a partial specialization of a member of a partial 
specialization, and we were confused into thinking that we only needed 
one template-header.  The parser.c hunk makes us generate the 
appropriate error.

The pt.c hunks fix the compiler to deal with it properly if it had been 
valid, since I expect we will allow explicit specialization of members 
of templates in the future.

Tested x86_64-pc-linux-gnu, applying to trunk.
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