This patch implements C++11 non-static data member initializers (NSDMI), 
as proposed in 
http://www.open-std.org/JTC1/SC22/WG21/docs/papers/2008/n2756.htm and 
specified by the C++11 standard.

For ease of reading, the changes are broken into four patches:

1) Implementation of non-static data member initializers with immediate 
parsing.  This patch was somewhat smaller than it might have been 
because there was still some code left from when G++ experimented with 
non-static data member initializers back in the early '90s, though I 
needed to fix a typo at one point.

2) Implementation of deferred parsing of NSDMI.  As with member function 
bodies and default arguments, names used in an NSDMI can be declared 
later in the class body, so we need to wait and parse them at the end of 
the class.

3) Implementation of deferred instantiation of NSDMI.  This is not 
specified by the standard, but seems natural given the other 
similarities between NSDMI and default arguments.

4) Implementation of core issue 1351 for NSDMI: discussion of this issue 
at the Bloomington meeting led to general agreement that an NSDMI that 
can throw should cause an implicitly-declared default constructor to be 
declared noexcept(false).  This patch is currently broken because it 
conflicts with deferred parsing (#2 above); we can't tell whether the 
NSDMI throws until we parse it, but we can't parse it until we've 
completed the class, and declaring the default constructor is part of 
completing the class.  So I've marked the test as XFAIL pending 
committee direction on this point.

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