C++ PATCH for c++/51553 (list-initialization using conversion op)

[Jason Merrill]

The code I put in back when I implemented list-initialization rejects 
additional user-defined conversions when considering the copy parm of a 
copy/move constructor, in order to avoid ambiguity when a nested 
braced-init-list could initialize either the copy parm or the parm of 
another constructor.  But this testcase shows that this is too broad; we 
only want to suppress the copy constructor when the argument is another 
braced-init-list.

That change fixes most of the testcase, but then we incorrectly accept 
the declaration of 'c'.  To fix that, we need to separate the handling 
of explicit constructors from explicit conversion operators in 
list-initialization.

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