C++ PATCH for c++/49107 (excessive instantiation due to noexcept)

[Jason Merrill]

49107 is an issue with the addition of noexcept to the standard library 
causing extra instantiations, to the point that code that worked before 
doesn't work now due to circular dependency.  Someone suggested that to 
avoid this problem we could defer instantiation of noexcept-specifiers 
until they are actually needed.  This patch implements that suggestion, 
and does in fact seem to fix the problem.

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