C++0x PATCH to add -Wnoexcept, fix defaulted fn exception specs

[Jason Merrill]

Two noexcept related changes:

1) We were failing to give in-class defaulted methods the same exception 
specification as an implicit declaration, as specified in the FCD.

2) I've added a -Wnoexcept flag to help identify places where adding a 
noexcept specifier would be helpful.

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