C++ PATCH for c++/58102 (constexpr and mutable)

[Jason Merrill]

We need to allow copy-list-initialization of constexpr variables with 
mutable members, too.  The thing we need to avoid is not so much an 
full-expression with mutable-containing type as assuming that a mutable 
member of a variable hasn't changed since the variable was initialized.

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