[PATCH] Add explicit default constructors where required by the standard

[Evgeniy Stepanov]

Hi,

according to n3242 8.5.6,

If a program calls for the default initialization of an object of a
const-qualified type T, T shall be a class type with a user-provided
default constructor.

This patches fixes 3 places in libstdc++ code where this requirement
is violated.

Please review.
I don't have svn write access.
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