C++ PATCH to implement C++0x noexcept

[Jason Merrill]

I've just checked in support for C++0x noexcept, as defined in

   http://www.open-std.org/JTC1/SC22/WG21/docs/papers/2010/n3050.html

A noexcept-specification is represented by putting the expression in the 
TREE_PURPOSE of TREE_RAISES_EXCEPTIONS.

Since most non-template noexcept-specifications, including 
instantiations of templates, will be either noexcept(true) or 
noexcept(false), we always use the same node for such specifications. 
This also simplifies some tests.

The first patch is some cleanup that is necessary for the noexcept 
patches, but is also correct by itself.  The second patch implements the 
noexcept-expression.  The third patch implements the noexcept-specification.

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