C++ PATCHes for c++/47453 (rejecting ({...}) for non-class types)

[Jason Merrill]

DR 1214 established that an initializer of the form ({something}) is 
only valid for an object of class type.  This patch implements that, 
though I've only made it a pedwarn as there is some uncertainty about 
whether this is actually what we want.  I've also split the patch into 
two to make it easier to revert if that turns out not to be the case; 
the first part is a cleanup which is correct either way.

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