C++ PATCHes for c++/54020, 54086 (minor constexpr issues)

[Jason Merrill]

54020 notes that we were accepting a constexpr function that is 
ill-formed, no diagnostic required, but it would be easy to give a 
diagnostic for.

54086 suggests that the standard seems to allow constexpr and const 
together.

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